As of today April 28, 2016 users will no longer be able to use third party search engines with the Windows 10 search assistant named Cortana.
Up until recently it was possible to use browser extensions and registry edits to set Google as the default engine for Cortana searches instead of Bing.
The reason for restricting users ability to use another search provider, such as Google, is because Microsoft have noted that doing so leads to a ‘compromised experience that is less reliable and predictable.’
Apple’s SIRI and Google’s Android voice search both lock the user into using their own search tools so this move is not that surprising.
The battle for search engine market share continues!