I only have one slot left now, but there are multiple new features I would like to add.
It appears that the answer depends on the program then. ss21 and above does work in affinity photo, but Apple’s own typography window does not recognise them.
The OpenType feature tag registry only gives us ss01-ss20, so ss21 would be a custom tag, rejected by many (perhaps most) apps. I’d suggest looking into the Character Variant features (cv01-cv99). I’m working on a font with a large number of variants, and these have been life-savers.
The variants will show up in the Adobe palettes if you keep the aalt feature up to date.