It doesnt matter much if the runway is on the sectional. What matters
is:
1. Who owns it
2. Who controls it
3. Who pays for it
4. Who uses it
5. Who doesn't want it

There are probably zoning laws. Is it allowed by zoning? Is there a
written arrangement for it's ownership and control? Five lots is not
many, but I have seen ones operate with fewer. How long has it been
there? Ultimately, you won't know for sure, I suspect. The only ones
that are "for sure" (is anything for sure?) are ones that have been
there a long time and are supported by many of the local home owners
who are active flyers. What seems to do these airparks in more than
anything is disuse.

Crested Butte has a local airport that is private, but public use
allowed. It was originally built to be part of a subdivision. The local
landowners dont want it and they are trying to shut it down. There are
many other similar stories. If there were several landowners that used
it and wanted it, it would be much more likely to stay open.

The one up at Red Feather Lakes is closed now (I think). It was the
last remaining posession of the developer. Not a single lot owner who
used it. Only about two pilots ever landed there in the last year. It
was on and off the sectional over the years. It was for sale for a
while. I am not sure what happened to the land. It might even still be
there.