I keep reading these references to Doom 1 and other 'first generation first-person shotters' as being "2.5D" To me this somehow implies they are 3-D perspective, but not quite up to today's "true ...

Then there’s Mega Man 2.5D.A fan adaptation of the classic franchise in its 8-bit heyday, the game feels like something salvaged from a dusty drawer in the Capcom vault.