mentat was archived by mozilla back in 2017, but there are a bunch of forks. Because github is dumb and has a terrible interface for exploring forks [0], I used the Active GitHub Forks tool [1] that helped to find:

qpdb/mentat [2] seems to be the largest (+131 commits) and most recently modified (May this year) fork of mozilla/mentat.

[0]: https://github.com/mozilla/mentat/network/members - Seriously, how am I supposed to use this? Hundreds of entries, but no counts for stars, contributors, or commits, no details about recent commits. Just click every one?

[1]: https://techgaun.github.io/active-forks/index.html

[2]: https://github.com/qpdb/mentat