I fully agree with Sören on all points, but with one nuance.
The 20/2.8 AIS is indeed remindful of mirror lenses in that it is most useful at one aperture, maybe two (f/8, f/11) for landscape works. The same is true of the 20/3.5 UD.
I found it acceptably sharp on the Df (16 Mp), after some processing in Lightroom (some sharpness push, and the autocorrection of the wavy distortion, obtained by using the settings of the 20/2.8 AF version). And of course, CA correction.
Once corrected in LR, one may be satisfied with the result, also remembering that the 20/2.8 AIS is one of the smallest and lightest lenses around - which is very useful for street shots.
In addition, it can be used at wider apertures for close-ups, where corner sharpness does no longer matter. A relatively seldom use case, but nice to have.
Bottom line, it definitely is one of my worst performers (alongside the 28/3.5 PC, "worst" being relative), but as the 28/3.5 PC, it is an enjoyable piece of optics on Df.
Attached photo shot at f/11, IIRC.