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Sites should be punished into oblivion for doing this. Why do browsers even allow it? Is there a legitimate use-case for this?

I am maintaining an SPA and the only thing I do is trying to not pollute history. But I'd never try to block the back button.

I think a legitimate use-case is asking for confirmation before leaving the page when filling out a form so that the user doesn't loose their data.

But this can be achieved with other means. I have exactly this on my SPA.