Are they? This seems about deceptive or malicious content (i.e., redirecting to ads) rather than “something in my history triggers a JS redirect”. I’ve definitely experienced the latter with MS, but never the former.

It seems like Google's policy is unconcerned with the intent of the practice. If a website JS redirect ruins the user experience by breaking the back button, it will be demoted in search results. It doesn't matter whether or not the redirect was meant to be deceptive or malicious, websites shouldn't be ruining the user experience.

> It seems like Google's policy is unconcerned with the intent of the practice.

I'm reading the opposite: "If you're currently using any script or technique that inserts or replaces deceptive or manipulative pages into a user's browser history that [...]"

This is Google. Most likely they will deploy an automatic scanner bot that "supposed to" handle all the edge cases. When it don't work, you will be blamed for not writing your js in the way the bot can understand.