@Stephen1105 Indeed there are Tier 3 restrictions due to some pandemic or other going on.
Advice is:
you should try to avoid travelling outside the very high alert level area you are in or entering a very high alert level area, other than for things like work, education or youth services, to meet caring responsibilities or if you are travelling through as part of a longer journey
you should avoid staying overnight in another part of the UK if you are resident in a very high alert level area, or avoid staying overnight in a very high alert level area if you are resident elsewhere
so I guess Airbnb are trying to apply said advice to bookings.