I mean, the SRD is pretty plain - there's a list at the beginning of stuff that is asserted not to be Open Content, and Underdark and Mind Flayers are on it, and in contrast the open content part includes names and stats for all the other races and items you mentioned. So it's pretty unmistakably fine to use those things, themselves, however one likes.
But obviously there's also a second layer where you can't publish your own novel about, say, the intricate details Drow matriarchal society, that aren't in the SRD and came from WotC novels and sourcebooks. Presumably it's a bit like with Sherlock Holmes, where the character himself is out of copyright but certain lore details added in later books are still covered.
But obviously there's also a second layer where you can't publish your own novel about, say, the intricate details Drow matriarchal society, that aren't in the SRD and came from WotC novels and sourcebooks. Presumably it's a bit like with Sherlock Holmes, where the character himself is out of copyright but certain lore details added in later books are still covered.