4 pane comic of dolan on the left and spooderman on the right
pane 1 (dolan): cum join opensurce cummunity!
pane 2 (spooderman): shure! how joyn?
pane 3 (dolan): Here discord! (with discord logo)
pane 4 (spooderman with tears in eyes): y u do dis?
IMO Discord is the best platform for this right now, which is unfortunate. The little I've tried Matrix has not been very impressive (single chatrooms, slow, bad self-hosting experience IMO), IRC is a bit better (though very dated in many regards, esp. user management) but still doesn't have the categories/channels that make discord nice. And most other chats are proprietary with discord just being the best one.
Which one would you like them to use?
Matrix is the best platform IMO, and actual dev communities agree. (See: Github, Mozilla, KDE, Nix, the list goes on)
This is in addition to forums, git, wiki, etc, which those communities also provide.