@Sam397 It's because the booked dates are past the current COVID policy dates- apparently Airbnb has been telling guests to send this cancellation message to hosts if they aren't eligible for a refund (no, all guests aren't getting 100% refunds- several have been moaning about it here on the forums). If the host clicks accept, it goes down a a host cancellation, with all the attached penalties and the guest gets all their money back. Really nasty stuff. Decline those messages. Then the guest has to cancel from their end.
This message has always existed, and hosts have been caught out on them before, because it just looks like you're approving that the guest can cancel, but in fact, it registers as a host cancellation. Savvy scammer guests try this option, and sometimes just guests who don't understand how to cancel from their end. But there seems to be a rash of these now, because I think Airbnb is advising guests to cancel like this if they aren't due a 100% refund.
Airbnb is doing this, of course, so the host will look like the bad guy instead of the guests yelling at Airbnb for not making sure they get refunded in full.