I'm excited for Linen[0] to potentially replace Discord for our use case because unfortunately it doesn't feel like there is another tool that we could use besides Discord or Slack. (We were on Slack for a long time but we just couldn't get people to use it, and we've had a lot more success building a community since swapping to Discord.)

What other tools do you recommend for Open Source communities to use?

0: https://www.linen.dev/ (They are going to be adding a 2-way sync soon that lets you post messages from the web into Slack, Discord, and even GitHub Discussions, so you can basically pick any platform and it doesn't matter. Kind of like the old IRC days!)

Sounded interesting but I'm immediately turned off by the wall of channels in their screenshot. They can't even show 3 letters worth of alphabetic searching in a full screen of text. I think discord has it's faults but they're working to reduce the wild growth of new channels and seem to cope with large communities in a somewhat decent manner.