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.

Actually, a warp drive is permitted by general relativity assuming it's possible to create 'negative mass': https://en.m.wikipedia.org/wiki/Alcubierre_drive

Of course, that's not a small assumption...

Apparently the designers of Mass Effect did their Sci Fi homework.

In my mind I imagine warp to mean actually warp space not fly through space, the theoretical Alcubierre drive may warp space. Space is shrunk in front of the Alcubierre spacecraft and expanded behind it.

The spacecraft doesn't actually move through space it pulls the space where it needs to go towards the spacecraft and pushes away the space behind it.

I'm no rocket surgeon so don't quote me.