There are entire Telegram groups devoted to publishing CSAM. They're publicly available, not E2E encrypted. Being there in such a channel puts the data on your device, unencrypted. You can report it all you want. Nothing happens. Been there, done it.

It is not just that. Weapons. Drugs. Terrorism [1]. Pornography depicting rape.

[1] Includes accelerationism. France just had a terrorist attack on a synagogue. Germany had one on a city festival. Could be related to either. We don't know!

And yet, with all above being said, as much of a cesspool Telegram is, I much rather have such centralized there than in an application with group E2E encryption. But even then, every once in a while you want to scare the herd to demotivate their (criminal) effort, just be careful not to flock them to a better alternative. Which is a real risk.