IRC:

#lisp @ libera.chat
#commonlisp @ libera.chat
#clschool @ libera.chat
#racket @ libera.chat

Reddit:

r/lisp
r/learnlisp
r/common_lisp

Discord:

Lisp: https://discord.gg/wQMQd27Ztu
Clojure: https://discord.gg/discljord
Racket: https://discord.gg/6Zq8sH5

Matrix:

#common-lisp:matrix.org

Mailing lists:

https://mailman.common-lisp.net/listinfo has lists for many different libraries and projects. In particular, the “pro” mailing list is somewhat active.
https://lists.racket-lang.org has Racket’s public mailing lists.