Один из основателей Signal набрасывает на телегу:

Telegram stores all your contacts, groups, media, and every message you've ever sent or received in plaintext on their servers. The app on your phone is just a "view" onto their servers, where the data actually lives.

Almost everything you see in the app, Telegram also sees

https://twitter.com/moxie/status/1474067550686236698?t=sZbCAadYGgBqkPej68wO6Q&s=19

Но вообще, лучше доверять проверенным encrypted messengers- таким, как ANoM.