Nrthugu

The ACLU has been a critic of the 13th amendment to the US constitution for some time, claiming that the clause that reads that slavery shall not exist "except as a punishment for crime whereof the party shall have been duly convicted" is a loophole and has been exploited as such.

After the Civil War, many states, mostly former slave states, immediately exploited the 13th Amendment loophole allowing slavery and involuntary servitude as a punishment for crime. Many former slaves were arrested and then put back into slave labor conditions through convict leasing, a lucrative practice that generated more than 70 percent of total state revenues for the state of Alabama in 1898. From the 1920s through 1941, convict leasing was gradually eliminated through state laws and by presidential executive order. The constitutional loophole, however, was never removed.

- Colorado Votes To Abolish Slavery, 2 Years After Similar Amendment Failed - NPR.org

In the bold portion above, the ACLU claims that "convict leasing" was done immediately after the civil war, was used to target and enslave black Americans, and was prevalent. Is this a fair assessment of the historical records?

According to The Iniquity of Leasing Convict Labor Engineering Magazine September 1891, pages 749-757:

The convict lease system exists in all the Southern States, with the exception of Maryland and Delaware. ... composed almost wholly of the freedmen fresh from slavery and the tremendous and, to them, wonderful results of the greatest civil war in history.

The reference says for example Tennessee has 1500 convicts, and that only Texas has more. This compares to a 1860 Tennessee enslaved population of 275,719.

See also Convict lease and Coal Creek War.

The word "many" here, being part of the OP's question title, is important, and the relative numbers as reported by DavePHD in his answer already addresses this part of the question. As to the whole statement, I would argue that, with "many" being understood as a relative word, the ACLU's statement is "a fair assessment of the historical records."

The core of the ACLU's statement looks to come from One Dies, Get Another: Convict Leasing in the American South.

There are various claims made in the ACLU's statement.

The first claim:

After the Civil War, many states, mostly former slave states,
immediately exploited the 13th Amendment loophole allowing slavery and
involuntary servitude as a punishment for crime.

The convict leasing system was a lasting force in the southern states other than Virginia until the 20th century:

All these factors-declining profitability, the presence of a penitentiary, the development of alternative means of using convict labor, and
significant political pressure against leasing, often connected with a willingness to undertake social expenditures-would be present in varying
degrees in each of the Southern states, permitting public opposition to
the practice, which was never far from the surface, to become effective.
But only in Virginia were these forces in motion from the beginning of
the post-Civil War period. Elsewhere in the South, they would not gather
enough momentum until after the new century had dawned. Virginia's
experience reveals what factors the post-Civil War Southern states would
have to develop before their systems of convict leasing could be overcome. No other state had such a combination of elements (except, in
radically different circumstances, Tennessee) before the twentieth
century.

The next claim:

Many former slaves were arrested and then put back into slave labor conditions through convict leasing

Note that the statement does not specifically make the claim that many former slaves were arrested in order to put them back into slave labor conditions. It simply claims that they were arrested and then put back into slave labor conditions. This is true in the sense that after the civil war the prison population's racial makeup changed rapidly:

(In 1866 Alabama) The convict population had also changed from the previous 99% white to the postwar 90% black.

History of the Alabama Department of Corrections (Thanks to DavePHD for the link from his comment to his answer, and to Aaron for pointing it out)

As to the phrase many former slaves, note that it would not have been at all profitable to have the number of convict laborers rise near to the level of the former slave population:

Slave exploitation has to be looked at in the aggregate because the
slaveowners’ maintenance costs must provide for the subsistence and
reproduction of the entire slave community, including the aged, the
infirm, and children; subsistence means more than simply the cost of
maintaining the individual productive slave. Lessees of convicts, by
contrast, were concerned in the main only with productive laborers.
The sick were refused or, in cases where it was possible, remanded to
jails. In January 1888, at the outset of the TCI lease in Alabama, the
company declined anyone ”who, though able to work, can do no more than
enough to pay for their maintenance,” as the exasperated convict
inspector explained to the governor. Later, in March 1893, the Sloss
Iron and Steel Company, a division of TCI, refused to accept Melissa
Stewart, who was nursing her infant, protesting that ”they had hired
no babies" and dispatching her to the Jefferson County jail.”

The final claim:

(Convict leasing was) a lucrative practice that generated more than 70 percent of total state revenues for the state of Alabama in 1898.

This number was apparently somewhat cherry-picked, given that over the next 8 years the state's budget surplus grew by a factor of 10, reducing the overall percent of state's revenues generated by convict leasing, but the number does appear to be factual:

In 1898 Alabama obtained 73 percent of its total revenue of
$378,120.48 from the hire of its convicts. The state spent $45 on
bedding that year. In 1901-1906 the state's total budget surplus
increased by a factor of ten over that of the previous five-year
period. Of course, the surplus represented more than the hiring of
convicts, but the governor reported that ”about as many [convicts] as
are physically able to be put in the mines are getting out coal by the
ton." Colonies of former convicts had begun to grow up around the
mines. One-third of the free miners at Coalburg were former prisoners
as early as 1889. The convict lease seemed insuperable.

From 1906 to 1910 revenues rose still more, from $1.2 to $1.7 million.
In 1915 receipts of the Convict Department constituted one-sixth of
the state's total income (although some of that revenue came from the
cotton mill at Speigner and from the prison farms, not from leasing
entirely).

In the bold portion above, the ACLU claims that "convict leasing" was done immediately after the civil war, was used to target and enslave black Americans, and was prevalent. Is this a fair assessment of the historical records?

First off, I'm not sure how to quantify "prevalent." Did it happen? Yes.

Here are some details for two states, Mississippi and Texas.

First, Texas. Moore is the person most actively working to preserve the legacy and history of the people used in convict leasing by Imperial Sugar Company.

In the years before the Civil War, Texas’s state prisons had held around two hundred inmates, all kept at a single facility, in Huntsville. After abolition, the prison population exploded, disproportionately with black men. Unable to house and feed all the new prisoners, the state began renting them out to private companies, who were grateful for the supply of cheap labor.

... He visited library archives in Houston and Galveston to look at so-called travel cards, which prisons used to keep track of inmates after the Civil War. "I would read those travel cards, and a big proportion of the blacks were in there on cases where they should have been doing probation or paying a fine," Moore said.

I wanted to provide those quotes first, then backup to a few paragraphs earlier in the article for a little bit more context:

Then came the Civil War. The South’s defeat and the abolition of slavery plunged the Texas economy into a depression. Deprived of their labor force, most of the sugar plantations on the Lower Brazos went bankrupt. One of the few that survived was the Williams plantation, which was purchased after the war by Edward H. Cunningham and Littleberry A. Ellis, business partners and Confederate veterans.

Cunningham and Ellis survived the abolition of slavery by finding a new source of cheap labor: the Texas prison system. Although they weren't the first growers to use convict labor, they were the biggest: in 1878 they signed a contract with the state to lease Texas's entire prison population.

I included this second set of paragraphs to point out this is "prevalent" in the sense that it's leasing all of Texas's prison population.

(I recommend reading the whole article, as it has other related details)

(Note: Texas stopped convict leasing around 1914)

https://www.texasmonthly.com/articles/sugar-land-slave-convict-labor-history/

PBS cites a book Worse Than Slavery: Parchman Farm and the Ordeal of Jim Crow Justice by author David M. Oshinsky about Parchman farm:

In the Reconstructionist south, states’ coffers were empty, prisons destroyed, and their former free labor supply was emancipated. Many southern states, including Mississippi, began arresting almost exclusively young, black men on charges ranging from laundering to larceny to murder. Some were legitimate, but many more were fabricated or embellished. The state began leasing the prisoners to wealthy contractors, who would further sublease them to companies.

"The convicts, under convict leasing, would do the kind of work that free labor would not want to do. In other words, the work a white worker might not want to do," Oshinsky said.

Under convict leasing, the inmates were essentially slaves again, Oshinsky said. They worked long hours for no pay, were poorly fed, and slept in tents at work sites doing dangerous jobs like dynamiting tunnels for railroad companies and clearing malarial-filled swamps for construction. Convicts, sometimes including children under age 10, were whipped and beaten, underfed, and rarely given medical treatment. Oshinksy writes that between 9 and 16 percent of convicts died yearly in the 1880s.

Then in the first decade of the 1900's Parchman farm opened, and convict leasing largely stopped in Mississippi.

https://www.pbs.org/newshour/arts/inside-mississippis-notorious-parchman-prison