See the discussion in https://git.reviewboard.kde.org/r/129741/

In particular:

I wonder if we could invent something more dynamic than the .protocol keys. An additional way to talk to a slave and ask if a specific operation (for specific URLs) is implemented. Something like KIO::CapabilityJob *job = KIO::capability(Move, url1, url2); connect; and in the slot, enable/disable the action accordingly. By default this would just use the information from the .protocol files, but in addition a new SlaveBase method (fake-virtual until KF6, using virtual_hook) would allow slaves to answer the query with more precision, depending on the actual URLs.