As far as I can tell, there are two de facto standards: highway=corridor and highway=footway+indoor=yes. It seems like more OSMers are familiar with highway=corridor, but highway=footway+indoor=yes has more examples in taginfo.
The main question is whether there should be a separate tag for indoor and outdoor ways, and how that impacts simpler renderers and routing engines. Personally, for now I tend toward using highway=footway+indoor=yes, which makes seems to me to fit better with Simple Indoor Tagging.