Is there a lemmy type engine, just for private correspondence? Retroshare is not like that. Osiris Serverless Portal is dead. And both of them are written in an outdated language (compared to rust) in C++.
may be related: https://лемми.опенсорс.рус/post/4015
Matrix?
Also c++ is not outdated it is still being actively developed. It is just not memory safe compared to rust.