HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\Windows\Explorer
If it doesn’t exist, and you’re stuck in the “Windows” folder, simply right-click on the right-hand sidebar and create a new key. Name it “Explorer,” without the quotes. Click on that new key in the left-hand sidebar, and then right-click again on the right-hand sidebar and create a new DWORD (32-bit value). Name it “DisableSearchBoxSuggestions,” without the quotes or the comma. Then, double-click on it and set its value to 1. Finally, reboot your computer.
Source: How to Disable Bing Search in Windows 10’s Start Menu