Earlier quoted context omitted.
I still do not understand why this is even allowed. It is so annoying. It is not even hard to do. Can happen accidentally.
I mean you have to be able to do history manipulation if you want to have history to any meaningful changes you do via JS on a page (and you should make sure to adjust the URL as well so people can go back to that information). But I agree, it should not be possible to prevent users from leaving back from the site.
You can easily spam the browser history using a redirect slide, effectively preventing users from leaving the page. No need for any JS, works even with active ad blockers. It’s relatively common on malicious sites.