Surely you could just... Use a different chat system?
Moving 100s of people out of a certain Slack server to a different chat system may prove untenable or outright impossible.
Doing that for a few Slack servers, especially ones where you're just another regular user, would likely be a full-time job, and a fool's errand at that.
#1: Much used, leading service went down the enshittification route. If we did this extremely convoluted geeky hack everything would be back to normal (it wouldn't).
#2: Let's use another service.
#3 and on: yes, here are other services which no one aside from statistical error uses.
Sigh.
Remember when people suggested to use Lemmy instead of Reddit?
You might ask, what's the point of this comment. Well, it's to get people to read https://pluralistic.net/ more and for the geeks to start unionizing before it's too late (it already is).
Particularly nice that the top article right now is a takedown of a company that screwed me over, so I might also be biased.
zulip's rather great tho.