I've never seen anything on the release relating to required depressurization planning. Usually the information on the release is about enroute alternate airports that are planned in the case of one engine inoperative and your drift down altitude will not clear the terrain ahead along your filed route of flight.

When you asked the dispatcher if the reroute was "terrain critical" how were you expecting them to respond? Did you specifically indicate that you were asking about critical terrain in regards to a depressurization event? Maybe the dispatcher thought you were referring to the grid MORAs. Or that you were questioning the drift down altitude capability with one engine inoperative and the need for enroute alternate airports?

Simply put, when you have a flight routing that puts you in the brown shaded areas of the Critical Terrain Orientation chart, always load the escape route in the secondary flight plan/RTE 2. Put the Escape Point into the PROG/FIX page and plot the 80nm ring around it. Don't expect the dispatcher to remind you to do this kind of planning, and they certainly aren't going to do any of it for you. Like CamYZ125 said, the charts have instructions on them. Use them!