Browsers have never worked that way, though. If GitHub was written back in late 90s, say, every time you clicked on a folder in the file tree, you'd get a page refresh with a new URL - and then the Back button would just go back to the previous URL.
Now, they do the refresh by fetching data from a web service and updating the DOM. But the URLs still change - as they should, since you are accessing different resources as you browse - and so the history tracks it accordingly.
The only time when we had something like you describe was in early Ajax days, when DOM updates were already done, but before the history API, and before the hacks that preceded it were devised. In those days, any website that did it would behave exactly as you described - the Back button in the browser would navigate off the website, even if you were clicking around it for the past hours. And then the website offered its own Back button implemented in HTML, that would navigate within it. Users hated it, because it broke all established conventions.
Now, they do the refresh by fetching data from a web service and updating the DOM. But the URLs still change - as they should, since you are accessing different resources as you browse - and so the history tracks it accordingly.
The only time when we had something like you describe was in early Ajax days, when DOM updates were already done, but before the history API, and before the hacks that preceded it were devised. In those days, any website that did it would behave exactly as you described - the Back button in the browser would navigate off the website, even if you were clicking around it for the past hours. And then the website offered its own Back button implemented in HTML, that would navigate within it. Users hated it, because it broke all established conventions.