CUE is keeping Slack for now and accepting limited history
https://github.com/cue-lang/cue/discussions/1839 has an overview of the situation and a discussion of alternatives being considered
They're a tiny section off to the side and you can't easily navigate between them, it's really badly done.
The main problem with alternatives is integrations, bots, and similar. The ecosystems are not as available, at least that is the impression
That's why someone was suggesting mattermost (iirc on a friday), as something that bridges all the chat platforms and gives you a single tool
If you read the CUE discussion I linked, you will see several people making this same point
1: I am not employed by CUE. I am their biggest fan and cheerleader
2: CUE has never paid for Slack
The context is that I have slack open, I am in 12+ groups. I've tried adding other platforms like discord and keeping them open.
The point is that, as a human, I only have so much attention, and having more than one chat program open at a time, in addition to the other messaging streams we are subjected to, it's a non-starter.
For my own sanity, only one chat platform can be regularly open. Thus follows the network effect and incumbent arguments
I noticed this as an independent web developer. When I charged very little, all my clients believed they could make unlimited changes to the expectations of the project, no matter how impractical or time consuming. When I charged what at the time I thought was way more than I was actually worth, someone still ended up paying me and they actually respected my expertise while leaving me alone most of the time.