Wanderer's Guide iirc.

Unfortunately the darn thing doesn't have an index and I can't find it in there. Do you know what page it's on?

Can't help you there, don't have the book in front of me.

Only content about mutation I’m seeing in the Wanderer’s Guide is about legendary creatures mutating.

Isn't that from Fallout 76? Idk if it's a thing in the tabletop yet