This certainly validates my instinct. I haven't used APFS on any drives that I intend to move back and forth between machines. By that I mean that I cheerfully

This certainly validates my instinct. I haven't used APFS on any drives that I intend to move back and forth between machines. By that I mean that I cheerfully use it on internal drives. And I don't worry about things like external time machine drives which are basically tied to one system. But I don't use it on a drive that I might need to take both directions between different systems on a regular basis. An investigation that would be more interesting, I think, than this version number probing would be testing what happens if you plug drives from systems with new APFS versions into systems with old APFS versions. Do the old systems take the drives into read-only mode? Is there some flag on the filesystem that indicates a minimum version that may write to the drive, which might make an older system mount the newer formatted drives read-only? That's not how I use Mac OS generally, but I could see it coming up in my house. My wife is happily using a 2015-ish MBP, which won't get Ventura and probably won't be upgraded for a couple of years given the light usage it sees. It's entirely possible (if somewhat unlikely) that I could find myself wanting to plug in a drive formatted by a newer Mac.