I saw a super simple example of this back in the 90s when I was first leaning web development (was in Web Development for Dummies or some such).

The page prompted the user for the password, and then used the password to generate the URL for the hidden page and redirected the user there. Of course this wasn’t over HTTPS, and you got a 404 if you entered the wrong password, but it was still a neat hobbyist trick for the era. I think the author even wagered that readers couldn’t defeat it.