[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Windows Search] "ConnectedSearchUseWeb"=dword:00000000 "AllowCortana"=dword:00000000
There ought to be an unofficial Windows 10 Settings app that uncovers stuff like this. Somehow Microsoft isn't quite getting it right and they ship Windows 10 with two different ones...
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Windows Search]