When looking for a slack alternative, please primarily look at matrix. It's open source (not open core or source available) and based on open federation. And it keeps improving at a nice rate.
And even it's still missing features.
Cinny (https://cinny.in) is the first Matrix client that actually looks promising. They just plain stole Slack's UX and it's working. There's no need to be unique for the sake of being unique when it comes to UX.
The clients though... whooo boy. Element is so bad that it hurts to use it. The worst part is that it doesn't have a singular major flaw, it has a hundred tiny issues that are just a bit wonky.
Mattermost is fully open source. I pointed to the GitHub repos which contain the entire client and server systems.
I really don't see the difference between open core and open source if the "core" is a fully functional standalone product.
That's the difference. Mattermost won't ever accept contributions, that reimplement features from their paid product. If you want to use such features, you would have to fork, and forced to keep the fork public, since it is AGPL.