When you join a channel and it says this "This channel can’t be displayed because it was used to spread pornographic content". Don't worry is just a minor restrictions for iPhone users. This is how you can get around it.
1. Open web.telegram.org
2. Log in with your username and password
3. The page will open just like you opened the telegram app but this is a web version.
4. Go to settings
5. Tick on Allow sensitive media (not allowing it means you can join adult channel so make sure you allow it)
6. Log out and restart your phone.
7. Now open the channel you wanted to join and you will be able to see all the adult contents
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