I mean... if you don't have an official term for it, but basically everyone in your field calls it a thagomizer, does that not make thagomizer the official term? If not, what makes a term "official"?
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