It could also be a hold over from the system prior to ACOM adding the living/deceased profile item.
Before that, if a profile had NO birth/death dates, they were publicly viewable (on a public tree). If a birth date was later added, the subject was now considered "living", but I don't think they would be removed from searchable database until the next indexing event.
Of course, if someone attempted to view the tree, they would now not be able to view that particular profile.
It may be that the owners of the tree with you in it have "abandoned" that tree. Can you tell the last time that user logged in or the last time the tree was modified?
One trick I used to use when I didn't know a birth date was to put a "dummy date" I knew to recognize (01 Jan 1980, In my case) and put "Unknown Birthdate" as the location of the birth (to remind me later).
That way, if the person was living, they didn't end up in the index and I could remember to later ask them what their birth date was.
This was really all a "belts and suspenders" thing, as my tree is private and non-indexable, so . . . .