I'm generally freewheeling, but I do have a few hard/fast rules.
1. Material from other setting books isn't allowed unless I'm running that setting OR special DM permission. I borrowed this idea formally from Dragonlance in that stuff from specific campaign settings should remain unique to that setting. No warforged outside of Eberron, no kender outside of Krynn, etc. If it's a setting that is multiversal (IE Planescape or Spelljammer) you pick a home setting and that restricts your options (so no warforged knights of Solomnia).
2. 3pp stuff is dependent on setting/genre. I allowed the bloodhunter and witch classes in my Ravenloft game, but I probably wouldn't in Faerun. They don't fit the feel. This is fuzzier as 3pp is too large to have an all-encompassing list on.
3. Generic D&D stuff (IE Tasha or Xanathar) is always allowed, but there may be caveats or additional implications. Again, I point to Dragonlance limiting what races are naturally occurring.
4. If a setting has a unique rule (Theros piety, Dark Gifts, Dragonmarks, etc.) they are only allowed IF the game is set in that setting.
My limitations are to keep games to a theme. My Eberron Pirate game was nautical, my current Ravenloft is dark fantasy/horror, and my players are expected to keep to theme. And for the most part, they do.