It goes back to bbcode forums, in particular Something Awful, which is ~circa 1999 (1998?). Your post would have an "edited by [user] at [N time]" which only shows up if the edit happened after the next post, or Y time had elapsed. If you made an inflammatory statement, and got dragged for it, you could try and edit the original post, and it would show that you changed your position later, and people would drag you even harder. A lot of people will quote you if you make an inflammatory comment so that you can't change the past. Their quote won't have an edit tag and yours will, after their post, which further points to you backtracking on your original statement. HN doesn't really use quotes to any great degree, neither does reddit.

Reddit came into being ~2008-9 so this practice of EDIT: blah goes back at least a decade before then.

yup, the reason why I number my edits EDIT, EDIT 2, EDIT 3 if there are multiple edits, so they match the number of edits in BB code forums, although nowadays you can't see the number of edits on Reddit AFAIK

though maybe it was already possible to edit FifoNet posts? not sure

> It's a habit carried over from Reddit users.

It's common (the use of “EDIT” to flag edits generally, not necessarily the practice of doing it for trivial edits), IME, in most discussion fora where comments are editable, and has been longer than Reddit has existed.

> It's a habit carried over from Reddit users.

It's a habit that goes back decades on e.g. web forums and editable comment sections; some forum software automatically marks a post as edited, sometimes adds a percentage how much was changed, and leaves an edit log for moderators. They add form fields so people can indicate what they edited, or add a grace period (setting) that avoids adding the 'edited' marker if edited within a set amount of time after posting (for e.g. typos).

TL;DR it goes back a lot farther.

Sure, but OP asked why the 'EDIT:' string on posts. In my 40+ years of online forum use I've only recently seen this habit on Reddit. I'm sure it exists elsewhere but this was my POV.

Most forums automatically add the edit string and the time of change and they usually provide a possibility to add an explanation. Here this doesn't happen so people add it manually. E.g. I have never had an account on Reddit, I do not visit it and I add it when it makes sense.

It most certainly does not.

It doesn't have anything to do with Reddit. It's about being honest and avoiding possible future confusion.