In the context of html in a browser, a form isn't a concept to be debated, it's a well understood element defined in the html standard.
If an app chooses to not use html forms and instead uses something like contenteditable to create its own editing environment, then you likely have much bigger issues than the back button.
I can't see how I have issues because a webapp I use wasn't perfectly coded. Remember the majority of browser users are, for a lack of a better word, browsers, not site developers. We can't affect the technology choices of the sites much, as choosing a site by tech purity instead of usefullnes is a losing proposition.
If I have a partially filled-out form that is really the query box for a search backend, and I hit back, why at the browser level should my flow be interrupted? That content was ephemeral.
It's an application-level question, not a system question. Can only be answered with semantic knowledge of the importance of the user input.
In the context of html in a browser, a form isn't a concept to be debated, it's a well understood element defined in the html standard.
If an app chooses to not use html forms and instead uses something like contenteditable to create its own editing environment, then you likely have much bigger issues than the back button.