it “exists” in the sense it wasn’t removed from old items that had it already, but can’t occur on new
mono (magick) class prefix was entirely removed in update 1.2, and rare dual class and mono class prefix on medal/amulet/chest/helm was redesigned
You can only drop magick mono class prefix if you’re on 9.8
drop tables for 1.2+ [Resource] Affix drop rates for rare MIs in a spreadsheet