[noja]

Because the user expectation is that a browser won't leak their private URLs to a search engine?

https://www.example.com/id/0ca6ade6b2bb1eea371d0b029f552cee/... may be "public" in the sense that it is accessible if you know the URL.

[chasd00]

isn't cases like this where the saying "security through obscurity is no security at all" came from?

[ghusbands]

Not really, no. That came about more from people claiming to have good security, but not disclosing their security practices and many of them turning out to be rather insecure.

Many products (Google Docs, Youtube, Office 365, Dropbox, etc) allow sharing things via unguessable URLs; it's a standard practice that was safe, until browsers and browser extensions decided it was okay to send private URLs to other parties.

I would not be surprised if the EU steps in at some point and fines them heavily for it.