Research. Are you wanting something similar to reddit/lemmy/kbin/mastodon, something like whatsapp, or something like discord?
I would say more like tg/whatsapp
Have you checked out Signal? It’s the most secure messaging app out there. Not even the US’ alphabet agencies can retrieve encrypted texts sent through Signal. It’s pretty much what whatsapp claims to be.
Other than the impractical task of reverse engineering then you can’t be sure what proprietary software is actually doing. Every update is more changes you’re not in control of. At best you can only say proprietary software has not been found guilty of “being a scam”, etc. When you have the source code then you (or 3rd parties) can find if it’s innocent.
The thing is I am looking at a private messaging/socializing app, there are some good comments with it on Mastodon, and I am thinking to create a censorship resistance group on there, but it is not open source.
You could host your own Matrix server
If you are reffering to Signal then both app and server are open source. Just that the server is centralised.
For community building, try checking out SimpleX. Its open source and has an interesting take on anonynity. Plus it doesnt require phone numbers to create a user identity.
As for either being censorship resistant, I am unsure how we’ll they’d cope. But my guess is they would perform just fine in most scenarios.
