I believe the edit button remains available until someone replies. I think that’s potentially a good idea or someone can reply to something which was in the post, and then the original poster can remove the thing they are replying to, potentially leaving the thread very confusing.
I use the edit button a lot myself lol, I’m a past touch-typer who isn’t quite as co-0rdinated as he used to be resulting in WAY more spelling errors than I used to produce so I regularly go back to either add more info or to correct a spelling or grammar error and I’ve noticed the button disappearing too – pretty certain about the reply switching off the option, at least that’s the conclusion I’ve arrived at over the months…
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."