Space is usually defined in terms, other than, a single dimension.

I know generally that to have island seating, guidelines suggest 15" overhang for the countertop and 24" linear seating space. I want to cut this to 21" width space. I am not really planning on using seating normally but want it just in case. I know it will be tight but has anyone else used a 21" width for stools at the island? How does it work for you?

It's an "island," not a peninsula, both ends are open.

24"x24" is the minimum width space that an adult can occupy. And you have to be pretty friendly to be OK with being crammed next to someone. These days, 30"x30" works much better.