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.
Telegram hasn't been secure since basically day 1. IIRC it went something like
Security experts: Never roll your own cryptography.
Telegram: We rolled our own cryptography!
Security experts: Don't. And it's broken.
Telegram: uhhhh.... We fixed it.
Security experts: It still looks really bad. Stop it.
Telegram: says nothing
Security is a spectrum. Telegram has never been the most secure alternative, but that doesn't mean it doesn't have any security.
From my first link