I think it has more to do with what kind of data they have access to at runtime - IIRC DLSS upscaling has only required the previous frames and motion vectors, so requiring depth buffers would mean it was no longer a "drop-in" replacement

> I'd also be interested in how post-processing fits in with this.

I think screenspace effects like film grain and tonemapping are excluded in the same way UI elements are rendered separately from the game.