The slot ruleset is actually a Eurocontrol ruleset, so it is not per country, but for the whole ECAC region. During the pandemic the use it or lose it rule was completely suspended, since last fall the ramp up to the normal rules have started with currently a 50% use it target. The only airlines loudly complaining are legacy carriers that seem to have a harder time filling up their aircraft (mainly the state owned Lufthansa group), while the LCCs are planning at 2019 or higher levels for this summer. The problem with suspending the use it or lose it rule is that then those state owned legacy carriers can effectively block healthy competition at absolutely no cost, distorting competition even more than they already do with their fat government wallets.
As far as i know the slots have to be used within the commercial operation of the slot holder, which requires AOC inclusion of the aircraft type used and equipment in line with their AOC ops spec.
The interesting thing is the fact that many of those "ghost flights" took place in the time where no use of slots was required. So the slot rules were not the reason to fly those ghost flights. What many do not realize that some airlines did not put their aircraft into storage to be able to ramp up very fast if that was needed. However, parked aircraft need to be flown in regular intervals, i did myself quite a few of those flights which, in the case of the A320 family, are required at least once every four rolling weeks. Those flighs usually take place under a ferry or maintenance callsign, but apparently can also use the specific slots that the airline holds.