I use anything I can get my hands on. Monster books, world books, character classes, spells, items etc.Have you actually used 3rd Party products at your game table in your last campaign or two?
(Books and PDFs, not personal homebrew.)
I've bought lots of 3PP content but none of it has been used apart from monsters...
-edit-
I excluded campaign setting and adventures as those can be pretty system agnostic.
Yeah, it's a third party D&D game for sure, legally forked off the 3E ruleset, although Troll Lords seem to be saying now that it never needed the OGL to begin with.I play Castles and Crusades. That counts I think...even if not used in conjunction with 5e or even another system.