I believe Trillian still continued to support other networks (as long as they could) after introducing "Astra" or whatever they called their own chat network. Most of them have died off over time or locked out 3rd party clients (AIM, Google, Yahoo, MSN, etc.).

I haven't used Adium in like 15 years but that honking noise will be with me for eternity.

Adium still works on modern macOS, in fact, I'm running it right now for IRC. And there's already an issue on github asking about M1 support :)