I like Slack as a product.
I hate how a lot of open source projects have adopted it as the de-facto community discussion forum, mainly because it's a product designed for companies/organisations - not a disparate band of interested contributors.
In my password manager I have 8 passwords for different Slack "teams" that I've joined, because you can't just have one account.
Being on some mailing lists, the most annoying spam is from people requesting an invite to the community Slack channel. Every day you'll get at least one or two people wanting an invite.
It's silly, but I don't see what the alternative is - Slack has built a product that people like, it's just it doesn't gel well outside a corporate environment in my opinion
Discord is actually fantastic for open source communities. React uses it. We use it at my company. It doesn't have the "8 different team accounts" issue you mention and there's this fantastic cross-protocol bridge called Matterbridge you can use to do n-way mirrors: