WeeChat
A terminal chat client started in 2003. The name stands for "Wee Enhanced Environment for Chat" and describes the ambition: not just IRC, but one interface for several protocols at once.
The architecture is buffers and windows, where each channel, private conversation or server log is a buffer that can be shown separately or merged. Plugins are written in Python, Perl, Ruby, Lua or PHP, and through them Matrix, Slack and Discord are reachable from the same terminal. The most unusual feature is the relay protocol: the client runs on a server and a separate front end — the Glowing Bear web client or a mobile app — connects to it. So you get bouncer behaviour and a graphical shell without leaving the terminal version.