Telegram encryption is not opt in. End-to-end encryption however is.
Also this always get mentioned and everyone confuses encryption and end-to-end encryption.
What seems to never get mentioned here unless I do is that there is more to security than end-to-end encryption or not:
WhatsApp would (will? I don't use it since years ago) happily upload your data unencrypted (actually unencrypted not not-end-to-end-encryted!) to the biggest data harvester of all -Google if you or anyone you chat with enabled cloud backups.
Signal had months I think where they had a weird bug were tje client would send pictures to people without the user triggering it.
Facebook Messenger besides leaking all your communication patterns to the second largest data harvester also have this nifty feature were if someone reports your message an unencrypted message goes to Facebook.
Facebook was also the ones that suggested people uploaded nudes so the could "know what they should remove", wasn't it?
Signal also had a nasty exploit that would let anyone who sent a specially crafted message take control over the signal users computer if they opened the message in the desktop client.
Telegram is also the only one that I am aware of that has reproducible builds for both Android and IOS. For every other client you have to trust them. With Telegram you can (could at least last I checked) check out the source, build it and compare it to the version on the App Store.
What I mean is not that one should trust Telegram (there are things I use Signal for), only that when it comes to security engineering there is a lot more to consider than end-to-end encryption and HN really struggles to see this.