@jwildeboer
Not really software, but I've tried running a project consisting of mostly young people with a mix of discord and github. Where they decide stuff on discord then someone (practically me) takes the conclusion and puts it in a repo

I'm pretty sure IRC/Matrix are a high barrier of entry for them, as they already use discord for everything. So in that sense, IRC is the locked down platform
Mailing are a bit different in the sense that everyone has an email, but it still requires some unusual forms of communication which isn't as easily migratable from discord

There is also a small use case where discussions must happen in private, so that goes against the full openness nature if your suggestion. For example moderation concern discussions. Which discord just so happens to have private channels to use. IRC/Matrix also do of course, but just wanted to mention a use case for locking down discussions behind a barrier.

(The use of github (and git in general) is also hard for them, but more so because they're not software developers, so it's not as applicable to the topic)