Looking at Golden State Killer and BTK Killer, there is evidence Zodiac is/was still unknown

Highly likely, IMHO, although of course we have a much better list of suspects than anyone looking for Jack the Ripper. There are still reliable census records, criminal records, living witnesses or the heirs of living witnesses around, and there's always the possibility of DNA or something interesting being declassified, it's unlikely but possible that new evidence will come up.

But that't the thing, there are good suspects known, but there have to be about a billion unknown ones. The search has looked at residents of the North Bay with criminal records, the possibility of the search being too limited has always been there. What if the killer was a resident but had no criminal record, and law enforcement never looked at him? What if he had a job that brought him to the Bay Area part of the time, such as a truck driver or corporate sales rep? What if he was a George Hodel type, who didn't want to do his killing close to home? It's really likely that he lived close enough to get the San Francisco Chronicle, but it's still avalable all over Northern California, if not further. The likeliest thing is that we don't have the right suspect, and we'll never know.