It's not a good idea to have multiple methods of communication in my
experience - we've had that before with GUADEC and it didn't work -
and you can set up your Discourse account to work like a mailing list
from what I understand.
My experience is according Kat says. Having more communication options induce a bit of confusion and lack of fluidity in the conversation. And Discourse is a great tool for talk (and fine for mailing list equivalente).
Considering many discussion should be managed through gitlab issues to add a third communication channel (the traditional mailing list) is too much enough :-)
