Well, I don't know if anyone else responds to the mailing lists anymore to be honest. You can still get in touch through IRC though. We are on libera currently(and dogfooding tox groupchats). #tox for user related questions and #toktok for development.
Edit: Also, development is happening on github: https://github.com/TokTok/c-toxcore
Related:
List of Tox clients:
What is Tox?
Tox library source code:
https://github.com/TokTok/c-toxcore
(Note that I am not interested in the security/cryptography aspects of Tox (if it is indeed secure -- which is questionable/unproven to me at this point in time) -- I am interested in the ability to write Chat/Voice/Video apps around the library (basically apps like Skype, Signal, etc.) To that purpose, Tox (and the tox clients) provide an interesting and potentially very useful source code base...)
Tox (clients include uTox and qTox) has an... interesting history, but is technically interesting (true p2p), and worked fairly well when I last used it (around 2018).