Microsoft has decided that Cortana, its personal digital assistant, is so useful that you'll never be able to disable it going forward. While Cortana has received a variety of upgrades in the Anniversary Update, and now supports multiple languages, deeper integration with calendars and applications, and can remember random facts about you, it can't be flatly shut off any longer.
Microsoft has confirmed that this behavior is intention and is the result of integrating Cortana directly into Microsoft's search box. With the Windows 10 Anniversary Update, the search box is now Cortana," Microsoft told PC World(Opens in a new window). "Customers can expect the same great search experience powered by Bing and Microsoft Edge with the added benefit of Cortana’s personality."
You can still disable Cortana's ability to collect information, stop her from collecting data about you, or connecting to various applications. Her behaviors, in other words, can still be controlled, including whether or not she listens for your voice, how much information she collects, and how personalized your own search results are. What isn't controllable, going forward, is turning Cortana off and just using a "standard" Windows search bar. From this point forward, Cortana and search are the same thing.
Currently, Cortana can be turned off via Gpedit.msc. Presumably the update breaks this option.
There are still some questions to how all this will work in practice. Presumably while Microsoft claims you won't be able to disable Cortana, you'll still be able to use Windows Search when you aren't connected to the Internet. The alternative, if you don't want to use Cortana at all, appears to be to periodically flush whatever information the machine has picked up about you and to use a third-party search utility.
The alternative, if you don't want to use Cortana, is to use a third-party search utility, continue using Windows 7 or 8.1, or switch to Linux. Alternatively, you can download a copy of the original Windows 10 build and then turn off Windows Update as soon as you install the operating system. The problem with going that route is that you won't receive any security updates, either -- and there's no longer any way to distinguish between updates you do want and updates you don't want in Windows 10.
How well this plays with users will depend on how locked-down Cortana can be in the Anniversary Update. It looks like all of the same features and functionality can be shut off, which would imply that Microsoft has basically renamed its search functions. If, on the other hand, Microsoft has stepped up the amount of information it relays behind the scenes, this could easily anger its user base yet again.
