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.
By lying aggressively.
Lying about being the first phone app with E2EE (they're not even close, by over a decade if we count J2ME apps) because Signal was called TextSecure back when telegram didn't even exist yet. Lying about their protocol, lying about their backup system (if you're using group chats or regular chats which are backed up they are visible to the admins and any other claim is a lie), bullshit propaganda against Signal, etc...
Oh and by the way, Signal has now finally launched usernames, so you don't have to share your phone number to use it anymore.