Yep, this is when it was called iChat. I used it for that exact purpose for some time. This was before Apple had their own protocol, though; I believe it just supported AIM and XMPP.
it's actually persisted more recently. I just checked a High Sierra machine. "Messages" has an "Add Jabber Account" menu option. An it's recent enough that it plays nice with all my existing more modern iPhone/macOS etc iCloud messages.
