First, there was ICQ and AIM. ICQ got bought by AOL, and they added compatibility between each protocol.
Then there is Pidgin (formerly known as GAIM) for desktop OSes (Windows/Linux/macOS). Adium is a macOS version of Pidgin, and Bitlbee is an IM to IRC gateway fork of Pidgin. Which means you would be able to access it with your favourite IRC client (possibly with the aid of Tmux/Screen). There were tons and tons of plugins for all these applications available, adding all kind of features, including adding of protocol support. I remember even different versions of protocols being supported. There was even a Skype plugin for Pidgin.
But Skype didn't quite like that plugin. Which brings me to a point: you may end up with fierce opposition from the developers or owners of the official clients, for a myriad of (valid & less valid) reasons. Using Pidgin with ICQ meant less advertisement income for Mirabilis/AOL (the ads were shown when a user would message another, in the message box). Skype/Microsoft doesn't like usage of unofficial clients. Moxie Marlinspike wants people to download the official Signal client from Google Play, and he doesn't want third party clients to use the Signal infrastructure. WhatsApp sued the maker of an unofficial client. Bnetd got taken down [1]. Unofficial KaZaA clients. Really, I'm digressing from IM now. The stories are endless.
Then there is Jabber or XMPP. This had backwards compatibility with previous protocols. Then there were clients which were basically a frontend for those protocols. An outstanding one was Nimbuzz.