I have been using and participating in other GNU distributions such as
Trisquel and Parabola, but mostly as beholder/viewer.

After more than 10 year the forum of Trisquel still active, not like 8
years ago before social media, but is active, and more important is used
mostly for new user, and non-technical user, also the Trisquel forum is
well indexed and when you try to find something related with Trisquel
the forums is the solutions for those issues usuall.y 

Trisquel also have mailing list but used mostly for dev/contributor and
the Irc for also for dev/contributors or more advanced users.

Parabola as well have a forum/Irc/mailing, but in this case the forum is
not quite active, maybe because the focus of parabola is more advanced
users, also as Parabola user as well I also check Archlinux
documentation to try to fix my issues or Archlinux forums because are
more complete.

So I don't think a forum crate fragmentation, and the Forum is an entry
place for beginners and unlike Parabola that has the documentation of
Arch, Guix need to have your own solutions and your own documentation
and a forum is a good place to build that.

So said that:
- Forum: A good place for beginner an non-technical user (I guess all
Guix user require some technical knowledge), also a good place for
create history and user documentation/solutions.

- Irc: For quick question, developer and contributor discussions and
  more advanced users (bridge to Matrix would be good).

- Mailing List: For contributors, developers, and more long-terms
  questions, as well more advanced users.

on the other hand I think that the mailing lists create a more conducive
environment for debate than the forum itself, but again, for new user a
Forum is a better place or to find quick solutions which on Irc are hard
to find.