Apparently WOTC does have a live registered trademark for Planescape - except the registration's word mark is misspelled as "PLANE SCAPE" for some reason. - The TTRPG network

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.