I know a couple who in early January were to travel from one European country to another via the UK where a couple of days would be spent. The incoming flight was to be with easyjet, the outgoing one with Ryanair. Both flights were operated but at this point, due to the emergence of the English "variant", countries were refusing entry to travellers from the UK. The couple understandably decided to avoid the UK rather than get trapped there in transit. They contacted both airlines to ask about a refund if they cancelled, pointing out the reason.
Ryanair refused a refund but the couple pointed out that if they travelled on the flight, the destination country would deny them entry and Ryanair would then have to fly them back to the UK. Ryanair still refused to refund. The couple said in that case they would turn up for the flight and present themselves at the gate. They asked what Ryanair would do, knowing that they had to make a choice: Take the passengers out and back, or deny them boarding (automatic duty to pay €400 per passenger under law). "I don't know" was the employee's honest answer. The couple repeated that they would come to the gate ready to fly.
To their infinite credit, easyjet's immediate reaction was to refund in full.
What Ryanair did not know was that the couple were never going to be in a position to turn up at the gate because they would be steering clear of the UK altogether. The matter was therefore never put to the test, which I think is a bit of a pity. The lesson learned is not to use Ryanair if at all possible - I know someone else who will rather pay (and has done on numerous occasions) for whatever alternative to flying with Ryanair is on offer.