I just got the notification for this, sorry. There isn’t a real official term other than tail spikes, but iirc thagomizer has been used in literature, such as this paper describing a new specimen of a stegosaur from Portugal and its impacts on the taxonomy of stegosaurs

Paleontology nerd here. There isn’t an official term for the tail spikes, but thagomizer is pretty much an unoffcial official term.