Matrix, and there’s a P2P one that just went around called PeerSuite? Both are far from perfect, but at least aren’t yucky corpo platforms. Sorry to come across so harsh in the initial comment, the tone in my head was light hearted I promise lol. More of a “I’m happy to help figure out an alternative communication stream in order to make me a useful tester.” Might be worth making your game a Mastodon account to direct people to from the Steam page? Could be a good spot to encourage people to learn about the fediverse and provide a channel for updates and a message system for testers? Idk. Discord is somewhat unique still in the type of organizational tools it provides, hence the love-hate relationship I have with it lol