Moved to Matrix, but there are active IRC channels on Libera as well.

The move to Matrix was not a community decision, which is why it's now fractured slightly between the different systems. Maybe someone will bridge them in the future.

I'm seeing plenty of Nix/OS/pkgs IRC regulars and seniors in matrix rooms, and lots of active discussion going on.

There is demand and mourning for IRC bridges, and I'm sure that will be taken care of sometime soon, but to me it seems Matrix is the new home.

I thought Matrix group sizes are limited to 1024 members. I can't google if this limit still exists.

There are no hard limit on matrix room member counts. e2e encrypted rooms with thousands of users se a lot of key negotiation traffic, but with such sizes maybe full encryption doesn't buy much as any user can share the log/history.

Umm, so O(n^2) key exchange traffic? In practice, is it noticable to users/ server admins?

O(N) per user for the initial key exchange when you join a room, and then O(1) thereafter - although you should re-key when someone joins/leaves or every N messages or X days (where N is typically 100 and X is typically 7).

If a "group" is a room, then this is definitely not currently true. I'm in Matrix rooms well over this size.