It seems really popular on DriveThruRPG.
Yeah, when the new Final Fantasy XIV TTRPG was announced, despite the OGL controversy that happened earlier this year the only TTRPG mentioned by name in the titles of news articles about it is Dungeons & Dragons (D&D). You would think it would cause a major shift in other properties being mentioned. Nope, it still seems it will be like the Mario Kart or Pokémon of TTRPGs, where people seemly instantly try to compare TTRPGs to D&D and mention the latter by name, but not anything else in the field, as if there were only two of that genre at a time or something.
I found no live trademark registrations when I search for "Planescape" trademarks on Justia's search and the USPTO search. So it was confusing to see artwork that shows a registered trademark symbol on the new Planescape book. But yesterday I found a website that claimed there was a "PLANESCAPE" trademark.
Apparently WOTC does have a live registered trademark for Planescape - except the registration's word mark is misspelled as "PLANE SCAPE" for some reason. (Word marks are supposed to be inputted as all capitals, I believe, when being registered as a trademark. The error is in the word mark's space when the logo and official marketing material spell Planescape as only one word with no spaces.)
https://ttrpg.network/pictrs/image/c09b2564-5e5a-4169-946f-7299c326e677.jpeg
So for anyone finding no live trademark registration results for "Planescape" (thus believing that book shouldn't have a registered trademark symbol on it), this must be why. Maybe you should contact their lawyer team and tell them that the word mark in the registration is misspelled.
Of course, I'm not a lawyer, so don't contact me about stuff like this. This was just an accidental discovery.
If I make a story about how a stegosaurus did amazing feats to the point they became blessed gaining feathered wings and flight, becoming the first dragon, then that's still a dragon.