I guess I figured it out (not sure): when selecting a person who is using iOS 16, you simply don’t see Check In, as it requires iOS 17.I must have missed something, but is Check In a USA-only feature, and if so why?
There is no information about this on the website of Apple, but it could make sense in the meaning of “don’t show what you cannot use, so we don’t clutter the array of options” kind of idea.