You haven’t watched Discovery, so I don’t want to spoil it for you (I don’t really consider this topic much of a spoiler because it wasn’t exactly integral to the story arc), so do not read on if you don’t want it to, but they most likely left the Federation after dilithium supplies ran out and the Federation fractured before Discovery arrived in the future. We don’t know that for certain though- at least I don’t think we do.
But that is really the most minor part of the discussion point, which is more about the practicalities of Vulcans and Romulans sharing a planet.