Honestly, this is one of a few areas where Discord on mobile is still buggy. I have yet to find a good way to do it I leave from the desktop. Sorry I don't have a better answer, but this drives me nuts.

It’s really weird because the servers that I am able to leave normally our bigger communities. I don’t think member count should affect anything but that’s the only correlation I have. Anyway, I have to leave through Safari, so I log into Discord and then I use the quick switcher there to get to the server I want to leave, then pull up the menu that lets me leave it. Hopefully this helps. Also this is a really weird trick but it works I’d say 60% of the time: when you’re in the menu that lets you leave the server (iOs app), do a three finger swipe like you’re scrolling, and then swipe back up to the leave server option. It should say something like page 2 of 2, and then just swipe back up to leave server.

Voiceover uses baby logic for a lot of these things. Basically, if a button isn't on-screen, it doesn't exist. This is the case with the button for leaving servers.

​

Once you're in the server settings menu, turn off Voiceover, scroll the server menu up a bit, turn Voiceover back on, and if you can find the leave button by touch, you should be able to activate it.