We need people working on open source protocols and tools to make things like this possible with existing protocols and in a secure way, federated/decentralized.
I saw in a comment below what I expected, it already exists LemmyBB!
Have you considered that some people don't see value in the decentralised/federated setup? They want to go to one site, that all there friends go to, without having to play guessing games to ensure that the server they are using is part of the federation that their friend is using.
It's also bad form to call out something just because it is proprietary: Ever used AWS, Azure or GCP products? Why is their "proprietary crap" better than this "proprietary crap"