Hey, key:virtual_tour is a new tag this is also currently in a proposal phase see https://wiki.openstreetmap.org/wiki/Proposal:Virtual_tour. The purpose of this tag is to introduce people to a particular place in a virtual way, in this case the royal palace in Brussels. See the url associated with the key: virtual_tour. Currently, I don't think there are any thirth-party apps using this tag. Hopefully once this tag becomes more widely known this will become the case. If you have any questions feel free to ask....