You're perhaps missing the point of this.

It's clearly not meant to be something you'd use regularly. It looks like an excercise of "let me see if I can do this"

If you're arguing about practicality, then you've already missed the boat by a few decades if you're running on this ancient OS.

I’m not seeing where Bitlbee supports iMessage. Is a plug-in needed for that support?

Bitlbee supports libpurple (Pidgin) plugins. That's how I message over Telegram with sic (tiny IRC client) against Bitlbee to people using smartphones around.

OK, but iMessage isn’t Telegram. There are Matrix bridges that support iMessage to varying degrees of success/features, but I’m not sure if the Matrix feature via libpurple would support whatever bridges you have configured inside Matrix. It seems like a lot of additional work/hypotheticals for something that is just a fun proof of concept.

Certainly there’s a lot of opportunities for improvement here!

Yep. This looks a bit clunky. Any old/rusty irc client (even the ones for DOS and a 286) will connect to public bitlbee servers, where you have several. By typing up 'plugins' at its &bitlbee channel (not a typo), you will get the supported chat protocols on that server.