"Zodiac killings"

I guess it's possible that they've IDed the perp, but he's dead and the evidence wouldn't convict in a law court.

To go back to the 60s/70s and convict, you really need DNA evidence or a confession.

Police in the UK do retain evidence in unsolved cases, for ever.

The oldest case solved by DNA was a 1956 murder case in Montana. Unfortunately the man IDed had died, but the family voluntarily took DNA tests which proved their father was the killer.

https://www.npr.org/2021/06/12/1005690930/detectives-just-used-dna-to-solve-a-1956-double-homicide-they-may-have-made-hist