I still think it's very poor design that the toggle is:

1 - Completely unlabeled, nor is the icon indicative/clear.

2 - Not sticky. It will turn itself back on, even though this is neither implied nor stated in the UI.

3 - The only way to actually turn it off is to dig into your search settings - what percentage of your users actually know that exists, much less think about looking there?

I'd argue that a user looking to configure the settings of their search, are likely to think about looking in the search settings.

On stickiness: search settings - turn off personalized results.

A UX lesson I learned long ago: If you have to have an option for it then it's a poor design

A UX lesson I just know: It is better to have options than unchangeable defaults.

Another UX lesson: do one thing and do it well. Apparently Google forgot that. I still think it is a great one. And yes, I know it pertains to the function software plays and web empires are a different beast, but I find the poetic irony of the down the rabbit hole fiasco being related to a *nix quib delicious.

I think as a professional it's incredibly useful to have little rules like this that provide a structure out of which to make quick judgements and decisions.

But don't become complacent and let these transform into actual beliefs. There is no quicker way to kill your creativity.