There are several indoor map viewer applications based on OSM data. Integrating their functionality into the main site would be nontrivial, since they require lots of custom code to run, but your idea of an unobtrusive indicator of the type of mapping in the viewport could be more feasible once we integrate Spyglass into the site.
In the meantime, for those who aren’t familiar with it, there’s a Data Layer feature, accessible from the Layers panel on the right side of the map. It’s more raw than the indoor mapping apps but gives a glimpse at all the different things we map in OSM.
Happy to discuss this further in a separate thread.