It was a many months transition, and it's finally done
Fun thing, you can actually make a backup of all* your messages, groups, contacts, etc. So before leaving you can have all of your data in case you need that one contact or something
The final red flag was as that allegedly Russian authorities were messing with people's deleted messages. Not for the first time there are news that they could read, modify, delete, see location, and etc. Screw it, this is unsafe, I'm out.
Also, these days telegram is really at the state of a pile of garbage, bloated, buggy, and shady messenger.
What happened with Telegram? I'm unfamiliar with those particular rumors.
... But also definitely not a fan of it in general. Their app has had terrible encryption (when it's even used) for a long time.
There have been rumors from its start. I have no idea of their validity. Like anything, it's hard to find the truth.
As for its encryption, while I dislike it's not open source, and it's deserving of some criticism, there have been no reported cracks of it that I'm aware.
That said, it seems to store your ~~public~~ private key on the server (though I'm not sure of this), which is not ideal, for sure.
There has been multiple breaks, like the good old 2^64 bruteforce attack when they used too short session identifiers, malleability issues that could let the server/hackers change your messages, reordering attacks, etc.