https://www.nytimes.com/2021/06/06/us/aclu-free-speech.html is one such source.
Note that the shift has been quite recent (more or less since 2017) and not uniform; the ACLU does still take up free speech cases, while avoiding others, depending on various things.
Related, but not mentioned in the above articles, are tweets from prominent ACLU lawyers calling for banning certain books (see https://reclaimthenet.org/strangio-shrier-free-speech-suppre... for a reference, but this was quite widely reported). On the one hand, said lawyers have a right to their own opinions on their own personal twitter accounts. On the other hand, the ACLU never bothered to say that this opinion does not represent the ACLU organizational position....