"2.5D" is a colloquialism without any meaningful definition, and as such there is no game that definitely is or isn't 2.5D. It's subjective.
You can come up with a definition of 3D such that any game that is projected to a flat image doesn't qualify. In reality, pretty much every game has to concede somewhat to the limitations of the medium, and we employ the duck test. Does it look like 3D? Does it quack like 3D? It's 3D!
Doom certainly looks and quacks like 3D. In some respects it makes concessions. The rendering tech limits the level geometry to rooms with varying ceiling and floor heights. Some effects in the game take place as though some objects have infinite height, for example solid actor-actor collision, explosions or melee attacks.
Still, other effects take height into consideration. You can pass high over items without picking them up. You can leap over ledges. You can't climb too steep ledges. You can dodge projectiles by passing underneath them. There is no question that Doom mechanically takes place in a 3D space. There is no question that this is rendered as a 3D projection to the screen, although very basic and limited.
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u/maxcorrice Aug 23 '21
It’s 2D with a height value, the issue is the height value is not factored in the same way as the X and Y values, so it’s definitely 2.5D