[yyyk]

"IPv6 privacy extensions...."

I recall these extensions are just optional. How many implementations actually implement these extensions? I recall Windows 10 had this broken for a year and almost nobody noticed...

[the8472]

They were broken in the sense that the preferred address would revert back to the permanent address. IPv6 still worked.

[yyyk]

If the address reverts to the permanent address, than the IP can still be useful for tracking, right?

[the8472]

Yes, that was the issue that was fixed.