There was a brief period of time where I was able to use IRC, Facebook and MSN from the same client: Pidgin
There are individual bridges out there that can fix that, though.
Because of it libraries for all kinds of services exist already. KDE Telepathy sounds pretty close to what you're describing (native (Linux) controls, multi window, no touch screen design, plenty of different protocols).
With the KDE project slowly seeming to move towards Matrix rather than XMPP I wonder how long Telepathy will stick around. It can still be a pretty good start for your own chat application, though, as large portions of the work have already been done for you.
An alternative approach I've seen people take is what I'm doing myself: Matrix/XMPP combined with various bridges. That way the chat client only needs to speak one specific protocol. It has its downsides but it does save you from having to develop abstractions over abstractions. You also get multi device support for free without having to scan tons of QR codes if you need to reinstall your client for whatever reason.
When I saw that I figured they were aware and would soon start blocking it.
The bigger they get, the less they want to interoperate.
It is great having a unified tool with minimal resource usage that is fully integrated into your platform.
Many clients including Empathy and Kopete allowed this at the time.
There a bunch of bridges[0] available for pretty much any communication network you can think of and some people even go so far as to use their Matrix client for email too.