I´m led to believe that he either died or that he was incarcerated for life after 1974 (being tha last option more improbable). For 2 reasons:
1 - When the Zodiac copycat, Heriberto Seda, commited his crimes and sent his letters, this was obviously publicized all over the country. If the real Zodiac was alive and well at the time, I believe his ego could not have been contained and he had to start again writing again in the early 90´s, to claim that the killings in New York were not of his doing and to insult the New York Z. Since there are no recorded letters of that time by someone else claiming to be the Zodiac (besides Seda´s letters), this leads me to believe that the original Zodiac was dead or roting in prison.
2 - On average, males have lower life-span then females, and IMO that increases the probability that he is dead.