Slack messages older than 90 days will be hidden for free users from September
slack.com
slack.com
It's going to be worse for small private instances but much better for large public instances.
For those small private instance, there might not be many messages left after the 90+ days are hidden.
I have no desire to use their product. It doesn't do anything new, except maybe the way the interface is structured. If that's so great, why isn't it immediately obvious?
This is the modern internet. Make a killer demo. Make it fast. Make it fun.
This is boring.
But maybe if you want something open source, consider forking an existing Matrix client rather than starting over?
What are people using today for technical communication besides Matrix (too cumbersome) and IRC (last line of defense)?
Is Discord structured in a way that would prevent them ransoming all your stuff years into a project like this?