On iOS. Once you scroll down so far the screen dims very slightly. The amount you need to scroll to make it happen varies from post to post, but it’s usually around 2 full screens.

See linked screenshots (forgive the iCloud link).

The dimming seems to be restricted to liftoff. Brightness remains the same in settings and the top status bar is unaffected. It can be reversed by scrolling back up.

Perhaps I missed it but I can’t find any way to list the communities on my instance. I don’t mean search or !all, just the list that you can get from http://myinstance/communities