* https://web.libera.chat/#bitwise (i.e., #bitwise on irc.libera.chat)
* https://app.element.io/#/room/#bitwise:matrix.org (i.e., #bitwise:matrix.org)
Both channels above are bridged together. So joining either one of them is enough to follow all conversations.
This isn't specifically targeted at professional mathematicians though. Most members come from a computing background. However, mathematics is a central topic here, with much of the discussion revolving around computer science and mathematical literature. Recent reading and discussions have been about topics like theorem provers (Z3, Lean, etc.), real analysis, Galois theory, distributed consistency, compiler and interpreter development, Standard ML, etc.
I got help with grad-level problems there. There's discussion there from high school through research, and is quite active.
[1] https://leanprover-community.github.io/ [2] https://leanprover.zulipchat.com/
This community played an instrumental role in my early days of beginning to study mathematics seriously. During its prime, weekly mathematics seminars [2] were organised by the members, which I thoroughly enjoyed. I still vividly remember the IRC user "TRWBW", who seemed to have an answer for every question, no matter how complex. TRWBW's ability to explain even the most challenging concepts with clarity and precision was very impressive. Sadly, he hasn't been seen in several years.
Although many of the original regulars have come and gone over the past three decades, the community continues to remain active. It still remains a good forum for discussing mathematics as well as asking and answering questions.
[1] https://web.libera.chat/##math
[2] https://freenode-math.fandom.com/wiki/Seminars#Past_Seminars