So, it's workspace and user/device specific: two of the workspaces I interact with regularly have these problems and the problems also show up intermittently for some users and not others. (Anecdotally, my experience is that Matrix/Element used to be annoying compared to the Slack experience and now I mostly prefer it to Slack)

I would be fine with the understanding that the IRC bridge was missing functionality (and it always was). Although threads might make it impossible to implement in a nice way now.

As far as new features go, I don't want any new features in Slack: it worked exactly like I wanted it to seven years ago and the new stuff is nice, but not worth the degradation in user experience.