No rule is inviolate
The term Underdark was created in the 1980s by TSR, so TSR owned that as copyright material until bought by Hasbro. They've specifically given notice that word is off-limits (for sake of the OGL folks to make their lives easier), so they haven't constructively abandoned it.Underdark has been trademarked or copyrighted? Thought it was a generic term.
It wouldn't stop someone from publishing an adventure that mimics the Underdark. But they'd have to call it something different, like the Depths.