Here is a quote from that article:

> You should treat all the tags on an object as a set of facts about the object, and prefix the keys of those facts which are no longer true as a result of the disuse

I think this prefix can be used for currency:XBT since this specific fact is no longer true (regardless of other facts), and it makes it easier to query valid-only places without checking for an unknown number of other tags (amenity, shop, craft, tourism, leisure, office, sport and so on). It's not my local area though so if you don't like this convention, let me know and I'll revert my edit