We used to burn witches and send children up chimneys and down mines. Thankfully, we now live in more enlightened times where these and other practices no longer take place, but there's little point in trying to change the past in the light of experience.
Being an IT bod, Alan Turing is a particular hero of mine. He almost certainly did more to win the Second World War than any other individual, with the possible exception of Winnie. But he was sent down for admitting to an act which was illegal at the time. The fact that it isn't now simply serves to show that we have made progress in the reform of social justice. A posthumous pardon is nonsense in this case because it implies that he didn't commit the act, not that it shouldn't have been illegal in the first place. Who next, then? Oscar Wilde? Presumably we'd have to ban the publication of The Ballad of Reading Gaol, as Oscar should never have had occasion to write it!