Fill in the blank with any open source group. Not picking on Mastodon, just using as an example.
Mastodon has used Discord since at least 2018. I found essential tidbits about how featured profiles originated for an article I wrote - that information did not exist anywhere else.
All of that history will be lost if / when they move somewhere else.
I wish there was more use of self-hosted forums, but I get the enticement of real time chat.
