My guess is that because it's possible to access adult material, Apple requires a higher age rating on the App Store than Discord would like.
For instance, both the Reddit and Apollo apps are rated 17+. There's nothing "adult" about them, except that it can access adult subreddits.
So by banning adult channels, Discord is likely looking to drop the age rating on their app. A 12+ or 9+ would be more appealing and potentially allow them a broader audience, especially when trying to target the teen crowd which may have parental filters enabled on their phones.
For instance, both the Reddit and Apollo apps are rated 17+. There's nothing "adult" about them, except that it can access adult subreddits.
So by banning adult channels, Discord is likely looking to drop the age rating on their app. A 12+ or 9+ would be more appealing and potentially allow them a broader audience, especially when trying to target the teen crowd which may have parental filters enabled on their phones.