I confirm that 2.5.0 is not affected but that's a minor thing, the feature is still broken. Note that in ZDoom you can set the quicksave slot using quickload and even regular save, that thing was condemned to fail.
Edit: it's the same in 1.23 beta, 1.22 still had the original Doom empty slots but instead of getting the error message it would perform a regular load.
Edit2: after reading the changelog now i think i know how it's supposed to work in ZDoom.
You can't select the quicksave slot with quicksave (unlike in original Doom).
For every session if you save the first save will select the quicksave slot, if not you must select with quickload.
So now i think it actually makes some sense.