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Whenever I see "Status Unavailable", that means that the object is older than when version comparison was first implemented (at least I think this is the cutoff, roughly), and has had no recent updates in one environment or the other.
Identical versions are listed as "Not changed", though as far as I know, if an object is too old it'll show up with the "Status Unavailable" status regardless of whether it's changed.
Thanks Mike. May I know what the date that comparison will be considered on the objects as old? I don't see any documentation or any other details mentioned about this.