Oh don't be so vague, go on and name one!
> How come no other messenger simply copies telegrams feature but with e2e?
I've been wondering the same thing, especially for something as young as Matrix that didn't (at the time Telegram was already good) have a decent client themselves yet.
I've also been thinking myself of doing exactly that, but then it's basically a full-time job to get something halfway decent. People worked on reverse engineering a server at all, not an encrypted server but just any working self-hostable server at all. I don't remember the repository name but it's on GitHub. Even this was abandoned because it's just a ton of work that very few people are going to use. Additionally modifying the clients to work with a different protocol is even more work.
So I can see why people aren't doing this because I'm one of them, but yeah what I don't get is why the folks at Signal/Matrix/Threema/etc. would rather start from scratch than do this.
It might be easier to have some sort of plugin that overlays encryption on top of Telegram and uses the existing servers. On F-Droid there's this project called OverSec <https://f-droid.org/en/packages/io.oversec.one/> that I've been meaning to try, though presumably it hooks on an input field level and so it couldn't decrypt images before decrypting so it's not a full solution. You'd need to really modify the client (every one of them) and everyone needs to use your custom clients.