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.
Justia works with US trademarks and law. In the US trademarks currently need to be renewed every 10 years - prior to the reduction in the late 1900s they appear to be needed to be renewed only every 20 years. I don't live in the UK, so the apparent 30 year renew requirement is much longer.
Also it seems Wizards of the Coast were somehow able to keep their Planescape UK trademark registration intact despite letting every other Planescape related trademark registration become canceled/abandoned. I'm not sure if they had to keep up with the trademark, or if registrations in the UK last much longer without having to be kept up to date. I guess they let it lapse at the time in the US for some reason, but when they started Dungeon Master's Guild, I think they realized they should have live registrations for the legacy setting names they had to have more power under the law to manage the use of them. I think a company would want to keep other companies in check when they're licensing certain IPs to others.