When you join a channel or group and it says "This channel cannot be displayed because it has been used to broadcast pornographic content " (This channel can't be displayed because it was used to spread pornographic content). Don't worry, this is just a minor restriction for iPhone users. Here's how you can get around it.
- Open web.telegram.org
- Login with your username and password
- The page opens like you opened the Telegram application but it is a web version.
- Go to the settings
- Check the Allow sensitive media box (if you don't allow it, you may join an adult channel, so make sure you allow it).
- Log out and restart your phone.
- Now open the channel you wanted to join and you will be able to see all the adult content.