I haven't found a multi-protocol IM that supports Slack without loading the Slack app entirely. E.g. Rambox just renders the Slack app.

https://rambox.pro/#home

I used to use Pidgin to connect to several communication networks via a common UI, even with the promise of having a single contact list:

https://pidgin.im/

I just realized that Pidgin may have a community-maintained Slack plugin!

https://developer.pidgin.im/wiki/ThirdPartyPlugins

We need a federated communication protocol, similar to email, where we can have real-time communication with people and groups on various networks and using differing software.