I talked about with Lutz Lange, member of the group who manage the website www.historic.place.
He think, but is not shure, the best way to tag is:
historic=stone
stone_type=?
But I’m not realy satisfied with this solution.
Do you have an idea how to tag these niches?
Perhaps you can show me some examples?
In the 18th 19th centuries, these sort of niches where often to see on the front doors of houses.
I’ve mapped similar objects as wayside shrine.
You will find a photo collection of objects that I mapped on Commons Wikimedia. AFAIK, a lot of the photos there were taken by me and thus also mapped in OSM.
For me a shrine (schrijn in Dutch) is a (small) place in which objects are placed for devotion.