I think Libera is (and only intends to be) restricted to catering to the pre-existing Freenode community; their sights are not particularly set on expanding/competing with newer platforms any more than the Freenode community's were. They just cater to people who like & want to stick to IRC, for whatever variety of reasons. I don't see any "growth" (nor "shrinkage") happening here beyond migration of users from Freenode.

Discord is not open-source, so I don't see how it's relevant; it's "communities" are primarily gamers & streamers. While there are some open-source communities based on Discord, the same goes for other closed-source platforms such as Slack & Telegram, and I'm sure the number of software development communities on Slack is (sadly) much larger than on Discord.

Of actual open-source options: Gitter, Discourse, Rocket, Mattermost, etc. - Matrix would seem to be competing relatively well.

Many people use Libera & Matrix interchangeably (although the bridge hasn't been perfect so far, but we're working on improving it). You can get to #foo on libera as #foo:libera.chat on Matrix and it should Just Work™.

Discord is... for folks who have got lost.

Libera retained like 90% of the freenode channels, with maybe 5% jumping ship to other IRC networks, mostly OFTC, and 5% jumping ship to Matrix, Discord and Slack, from the posts about channels leaving freenode. Libera is also bridged to Matrix, so Matrix users can participate in Libera channels, so there's also a decent amount of people using Matrix as a glorified bouncer.

Very few new communities are setting up on IRC. My impression is that until the Freenode meltdown, Discord was most common, followed by Matrix, then Slack, but the freenode disaster seems to have benefited Matrix more than Discord or Slack.

That's if you look at official channels, if you look at user counts in unofficial community groups as well, Discord has to be way in the lead. The Python community discord has about as many online users as Libera in its entirety, for example.

Others have mentioned Discord and Slack being closed source, but I'm interested in Zulip as an alternative. I really like the "branch off a topic at any time" that leads to proper distinct threads of conversation that get much easier to follow. And the interface so far seems pretty polished and seamlessly responsive, with rich keyboard shortcut interface too (which I'm yet to learn tbh). I hope it too continues to grow as a good FOSS option.

but Discord is closed source...

I think IRC vs. Matrix does not exist. There is a certain decision what is going to be the main communication source, but then you are going to bridge whatever you have on IRC to whatever you have on matrix.