I was under the impression that AIM, Yahoo Messenger, and MSN all used XMPP. It seems that AIM added support for it in 2008, but Adium (and the other multi-chat clients) were just doing some magic to make it work seamlessly.
Definitely not. AIM, YIM, and MSN all used their own proprietary protocols, and multi-client messengers like Adium contained implementations of those protocols. There was no messaging between services (except AIM <-> ICQ), and some of the services supported features the others didn't.
Sure there wasn't messaging between systems, but the point was I had friends on AIM, YIM, and MSN and I could use one program (with three accounts signed in) to message them all and not have to think about it. Adium would just aggregate them together, so I'd see "FirstName LastName" and it would message from the right service.
But modern chat apps like FB Messenger, Google Chat (if it still exists?), etc. managed to successfully break all that, never worked reliably in Adium or Pidgin (if at all).
I remember I had a pluging for Trillian that allowed me to write code to script it in Tcl. And then a plugin written in Tcl that allowed me to quicksearch my contacts. Good times.
