I am sure that the Soviets had plans for the tunnels. Unfortunately for the Russians, most likely they were probably kept in a local office in Mariupol which, like the old Soviet nukes in Ukraine, would have fallen to the Ukrainians upon their independence.
That’s possible. But those underground facilities (established with the idea of surviving nuclear war) at the steel plant were designed and constructed (as best as I know from limited reading) during the times when the Soviet Union was a CENTRALIZED top down command economy. My suspicion is that somewhere in the former Soviet Union’s successor states (most likely Russia) there was centralized information on ALL of the Soviet Union’s major underground nuclear shelters. Of course, that’s only my conjecture. And it’s always possible that upon the dissolution of the Soviet Union entropy set in and such important information degraded or was discarded.