I have a floor-aware map that follows the schema/configuration found in the help documentation for configuring an 'indoors' schema.
Field Maps recognizes the schema and presents the floor picker options.
Yet - in the Map Viewer - there are no floor options presented.
This documentation: https://doc.arcgis.com/en/arcgis-online/get-started/view-floor-aware-maps-mv.htm makes no reference to ArcGIS Indoors licensing (yet following the link to 'Floor Aware Maps', there is a note that ArcGIS Indoors licensing is required).
Is an ArcGIS Indoors license required to have a 'floor-aware map'?
I believe that an Indoors license is required. In order to define a map as floor-aware, in Pro you need to go into the map properties and setup some things in the Indoor Layers tab, as described on this page: Configure floor-aware maps. If you don't have an ArcGIS Indoors license it seems this tab will not appear. Same for additional layers - there is supposed to be a Floors tab in the layer properties: Configure floor-aware layers
Esri have released a custom Floor Filter widget on Github, which could be used as a workaround: https://github.com/Esri/indoors-floor-filter-js