YES to `building=grandstand`.

Do you want to update them?

- - -

Random Note: I didn't even know that "grandstands" existed as an option (until very recently)!

I only started OSM about 2 months ago, and definitely wasn't mapping big stadiums. (Mostly just baseball/sports fields in small parks/schools which only had bleachers.)

Usually when I arrive at bigger stadiums, they were already mapped!