I would love to know the answer to this question.
There are things I've said in discussions I've had here that were not always well informed or motivated by my better self (yeah, you're free to peruse my comment history and see). Or I may have just grown up a bit since I said them. Either way, I've always felt it a bit unfair that they could be dug up years later.
I think there is some merit to the idea of Right to be Forgotten[1] but at the same time, I feel like that would just help evil people hide their true nature. The other half of me wants to just have the NSA dump everything one day and let everyone know what everyone else has really been saying for the last 15 years. The first couple days would be tough for everyone to acclimate knowing who everyone really is. But in theory that is a net good. Deception is rarely for good.
I doubt there's anyone who decides "Imma be evil" or realizes it of themselves to such an extent. If you believed you were evil, wouldn't you want to change it? Even terrorists don't think they're evil.
Maybe another approach to all this could be that, once you post, you can delete for a while shortly thereafter. But then your post goes into a period of extended residence on a site. Perhaps a year or something. After that residency period has expired, you're free to delete the post.
About a decade ago I was involved in building what became for a short span one of the best cinephile discussion boards on the Internet. Aside from HN, or maybe MeFi I can't think of any forum that sustained a signal-to-noise ratio of that quality for that long. It was night-and-day better than IMDB boards.
Naive and idealistic as we were, we have users full control to edit/delete posts. Over time we tightened that to have time limits, etc. But there was one big loophole which was you could delete your entire account and it would purge all your posts. Eventually one of our most prominent members with thousands of posts decided to (╯°□°）╯︵ ┻━┻. The result? Utter incomprehensibility of half the threads, effectively rendering the history useless if not downright misleading.
I get that the pendulum has swung too far and users deserve more control of their data, but a public forum is fundamentally a different thing from a social network—a forum thread is different from a post in that the value of the thread is communal, it should not be owned by the individual in the same way that it makes sense for a social media post to be. And even in that case, I think most of the problem comes from the giant many-tentacled nature of Facebooks and Googles where the data doesn't even come from directly entering it in and it's incredibly difficult to even visualize what they have. For a simple forum I think it's perfectly reasonable (and clear!) to have ToS that state: you post it, it's here forever, barring special requests due to extreme circumstances or legal reasons.