Interesting I had no idea. So, there’s groups that would work on the web client presumably but not the app. What happens if you join that group using your browser and attempt to see it in the app?
https://upload.wikimedia.org/wikipedia/commons/2/2a/Telegram...
The exact message you see is "Unfortunately, this channel couldn't be displayed on your device.". It doesn't say anything about the app store because apple policies don't allow apps to say that contents are censored because of apple guidelines.