From what I've seen and read on here, most of the rejections from active providers, assuming you're not a POS to providers in the past and the provider isn't a known flake, is usually race. My friend has been rejected twice after mentioning his race (Asian) and those same 2 providers would have no issues seeing me (white). However, it's still pretty rare.
I avoid any and all providers who publicly put "No AA" at all costs first and foremost. They're the type of providers I want nothing to do with. However, it's tough to know which some providers have that mindset since not all of them advertise it (i.e. I've heard rumors that Brandi Love not only does not see black clients but any and all clients who aren't white, but I can't confirm for sure). Hopefully someone makes a thread of providers who explicitly don't see other races so I can avoid them.
Other providers I've been told and have read on here and in past forums like reddit who discriminate based on race:
-Allie Nicole
(Porn Companions) who allegedly only sees white clients
-Veronica Rodriguez and Darla Pursley both rejected my friend I'm 99% sure because he was Asian. VRod saw his application and just straight up said "No." It was super unfortunate given how she was one of his ATFs.
Also all of those providers who discriminate but want to legalize providing will have a rude awakening when legalization will mean it's illegal to discriminate any clients based on race lmaooo.