deleted by creator

You can reuse the ‘recyling’-features for this. Add recycling:flags=yes as extra tag. On openstreetmap.org, if you click (or create) a feature, you’ll have ‘Tags’ in the sidebar, it’s the second expandable area. Add recycling:flags in the left column and yes in the right one.

There are four such boxes mapped already

Well, it’s OpenStreetMap, you can basically tag anything. Although I’ve never seen something like a dedicated flag disposal here in Europe, you could combine amenity=waste_basket + waste=flags.

waste=flags is only used twice to this date, however. See taginfo.

How this data is used, however, is up to the renderers/data consumers.