My missing Art lens list is:
Sigma 12mm f/2.8 DG ART
Sigma 20mm f/1.8 DG ART
Sigma 24mm f/1.4 DG MACRO (1:3-ish) ART
Sigma 85mm f/1.4 DG ART
Sigma 135mm f/1.8 DG OS ART
Sigma 100-300 f/4 DG OS ART
Missing in this list is the 28mm, or is 28mm a dead prime focal length?
As a prime shooter, I don't really miss anything in-between 24mm and 35mm FLs . Sigma has the old 28/1.8 DG Macro in their line up, which isn't that great on FF (actually, none of the 3 are). If I remember correctly, they released all three (20/1.8, 24/1.8 and 28/1.8 ) at the same time some 15 years ago. So, if they decide to remake all three lenses for their Art lineup, then 28mm would get the least of my attention, unless it would be superior optically by a reasonable amount. For me, something like a single 21/1.8 DG Art would be perfect .
Actually the 20mm is a rear focus design and it doesn't change length when focusing. That makes it a bit of a different beast from its 24 and 28 mm siblings that have a front focusing design. I'm really satisfied with my copy by the way - it's a solid piece of glass that interestingly can generate quite nice bokeh when you use its near focusing ability.