It's worth noting that there never have been any doors in the Tau Dewa Sector Block. So it's not that Romulan missions are denied access to them, it's that they're not there in the first place.
True, but there are also a lot of missing sectors from the other sectors that used to be main Romulan space. Iota Pavlonis and all those green sectors. Right now, the only thing we have in the traditional green of the sector map is the surface of New Romulus.