My understanding is that it's not forbidden (wormholes would explicitly allow FTL travel, for example), but that in Einstein's model, FTL travel or communication would break causality, because it would effectively allow one to send messages into one's own past.

That may seem like "equivalent to forbidden", but some of the interesting surprises of quantum physics were originally theorized by Einstein and company as a way to disprove quantum physics because they were clearly too weird to be true. Maybe not as weird as "Oh, you thought an event's cause had to precede its effect? LOL.", but bizarre and non-intuitive nonetheless.