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.
I don’t think this is a mistake.
When the setting was first released it was styled Plane Scape and though has subsequently been used as Planescape the registration may date as far back as the initial early 90s.
Trademarks are a funny thing though, take Coca-Cola. If they register only COCA-COLA with the hyphen, that doesn’t allow Pepsi to make a sparkling brown sugar drink called Coca Cola. Intention is important in these matters, not just the technicality of what is registered. Registrations also allow for “stylisation”, which means you don’t need to register a new mark to stylise your existing mark.
There were different registrations, but they were canceled or abandoned. The only live one is from 2016...