The amount things you can map in OSM is impressive. However I ran into two things that have no formal tags: time capsules (historic=time_capsule), and camera obscuras (camera_type=obscura). You can add the values, but they are not official. Time capsule is in the work [0].

[0]: https://wiki.openstreetmap.org/wiki/Proposal:Time_capsule

> However I ran into two things that have no formal tags:

You can use https://wiki.openstreetmap.org/wiki/Any_tags_you_like on OpenStreetMap. Many of the used tags are simply "in use" and not formalized with a proposal.