Slot Preservation - minimum requirements
 

In many countries in Europe, airport slots are on an 80% use-it-or-lose-it basis
While this has been suspended due to Covid, this suspension won't be renewed forever

Apart from complying with an airport's general rules on what can fly (eg no props or no single engine aircraft) and also considering any turbulence generated from large aircraft using a runway in the previous minutes, are there any rules as to what aircraft operates a ghost flight and where it flies ?
Could a large airline swap in a leased 3-seat (yes, 3 seats) old and knackered aircraft to fly to an aerodrome 5 mins away just to keep the slot nominally "active" ?

The only real constraint I can think of is public relations - namely avoiding criticism in the press by the environmental crowd and preferring to operate a commercial passenger flight at a small loss instead of an empty puddle jumper

Well, they are already getting hammered in the press for this. Needs someone at the top of the pile to set a further period of suspension for this regulation.

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.

Thank you Dentithat is interesting and helpful. As always, the detail is lost as it travels through the media. So much of today's news is simplified to a ridiculous degree. On a ghost flight, if there are no pax, will the carrier take freight, or do they have to be empty?

Well, "ghost flight" is not a defined term, so it is whatever the airline makes of it. Maintenance for parked aircraft were mostly just traffic patterns, so simply take off, fly around the airport and land. Usually around 15 minutes of flight time. But of course they were sometimes used for other stuff as well. We used them to transport crew to and from training. We are limited to maximum 19 passengers, since that means we do not mean a cabin crew for that or cabin crew on board is not paid for active duty (just deadheading which is cheaper for the company). Since i work for a LCC we never carry cargo to begin with. Friends in other airlines told me of flights with 20 to 40 passsengers in an A350, but a full cargo flight. So that could be justified as "ghost flight", but since the cargo rates are way up it was probably somewhat profitable.