I think it’s important that the Rust project picks a platform and sticks to it, the fragmentation is certainly a pain point, but I think it’s been (and is) worthwhile to have a period of experimentation.
I think Discord is a reasonable choice, it’s stable and has good clients. Though, I do think that Discord’s threading model isn’t conducive to productivity - it’s easier for conversations to get lost or become hard to follow. This becomes particularly evident as the number of participants increases (which it will much more than IRC due to a lower barrier to entry). I also have reservations regarding it being entirely proprietary, although I understand the pragmatism argument.
I’m also not a fan of IRC, for all the reasons that are usually mentioned, so I’ll avoid going into them. I also don’t think any of the other options I’ve seen mentioned are viable - Telegram, etc - I believe that any tool that isn’t designed for a “workplace” sort of communication would struggle to scale and remain productive (though, I certainly could be wrong).
I’ve been using Zulip a lot as part of the NLL WG and I’ve found that it’s an excellent tool for communicating and keeping track of what’s going on. I’d be extremely disappointed if the NLL group moved away from Zulip (though I’d understand if it meant all WGs were in one place). Zulip has rough edges, it’s mobile app needs some love, but it’s really great once you get over that initial unfamiliarity and I’d urge other teams and working groups to try it out for a few weeks before making a decision on their preference.