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.
The UI was also very fast and transparent -- not a lot of stuff separating somebody from the other people in their conversations, which was pretty solid even compared to other messaging apps of its day. Most people didn't feel the need to fact-check its privacy and security claims because it worked good enough for them!