It looks like “Selective focus” functionality in Windows Photos (photo viewer) becomes unavailable when Windows Photos is updated to v16.1111.10000.0.
By the time that I find simple way to do the same, I tentatively use GIMP instead. (I uses Japanses Windows, thus, my guess of English expression may not be correct.)
This is just a tentative measure. The boarder of the selected range looks unnatural. Futhermore, since the blurred amount is the same all over, the blurness looks artificial.
I hope somebody let me know better way 😀