ATS2 has been effectively frozen for a number of years, the recent commits have mostly been build changes so it can compile ATS3. ATS2 is stable and usable today and release notes are in the CHANGES-ats2 in the repo. I have a few posts showing some of the features on my blog (https://bluishcoder.co.nz/tags/ats/) for ATS2 and the previous version, ATS1. ATS3 is under development. It compiles itself but from what I can tell it is not generally usable at the moment. It's main design change is two-layer type checking. ML-style first, dependent types on top, in an attempt to lower the learning curve. The ats-lang-users google group is probably the best place to follow progress.