What people/deities and places fall under the SRD and how much wiggle room is there?

So i’m working on an original campaign setting but using 5E rules, and in the core and extended rule books it references things like "the Raven Queen" and the "Shadar Kai", "shadowfell and "fey wilds". none of these are specifically listed on the SRD under the IP lists, but it does mention "specific places". anyway, my ultimate question is, can I used IPs such as Raven Queen, Shadowfell, fey wilds, ect. even in an original campaign setting/world, so long as it’s using 5E rules and if I give credit to WotC? what falls under OGL, SRD, and general use? this question came into discussion by a player, who want’s to be a shadar kai, Raven Queen warlock from the shadowfell, all things that would require me to write them into my campaign setting, and they are arguing that these things being playable, should mean that they can be in the world.

the OGL and SRD are SUPER vague about all these things (obviously trying to give them legal wiggle room to sue whoever they want) does anyone have any further information on what is useable?