Pidgin deserves a mention - 24 years and still under development. The project has been undergoing a huge rewrite for 3.x, which has stalled visible progress somewhat, but I'm really hoping it will be worth it, as I'm excited to see it supporting modern IM features with protocols such as XMPP.
The lead developer regularly streams live coding sessions and quarterly project updates: https://pidgin.im/post/
The lead developer regularly streams live coding sessions and quarterly project updates: https://pidgin.im/post/