Protection
These days it’s advised to leave the house only with a face mask, to try and stay protected from germs. These plastic cones are usually used after a cat or dog goes through some sort of medical procedure and must avoid licking their faces, and these days we’re advised to avoid touching our faces as much as possible.
It may well be that this cat has found a new means of protection when he wants to step outside and get some fresh air, or otherwise that he’s adopting a new cat fashion for these new times. Whichever it is, judging by the look on his face, he seems very serious about it.