No? It's more like a BB with inbuild live voicechat, streaming etc.

IRC itself doesn't provide a shared history, threading etc.

You could get there with alternative clients and bots (at least if all participants use the same), but calling that IRC is a stretch

threading isn't particularly impactful if 90% of discussions don't use it.

Discord reuses the IRC "aesthetic" with "servers", channels and slash commands, but the actual structure of the conversations is much more like a forum.

In what way? It’s still time based instead of topic based.

You can turn any top-level message in a room into a Thread. I don't ever really see that used though, and I don't believe it bumps the thread when there's a new message (then again neither does Reddit).

I think it depends how the server is structured. You can also have "threads" which are topic-based inside a specific channel.