Microsoft has disabled the F8 key so you can no longer boot Windows 10 in Safe Mode right after powering on your computer. Safe Mode (although it's no longer called that) is still available but is only accessible if you explicitly asks for it when running Windows (there are plenty of articles that explain how this works). Windows 10 also has a mechanism for detecting boot problems and will automatically start in Safe Mode if normal startup is no longer possible. But what if you're not able to start Windows 10 the normal way, and the detection mechanism doesn't work? You can then opt to boot using a Recovery Drive (can be a DVD or USB stick). Or, you can simply re-enable the F8 key... (needs to be done before the problems start, though...)

Window 10 introduced a new policy in which it no longer waits for the F8 prompt. In order to allow Windows to wait just long enough for the F8 prompt, you can change the boot menu policy so it uses the "legacy" settings, including a slight, almost unnoticeable delay, but re-enabling the most desired F8 key.

To enable the ability to press F8, run the following from an elevated command prompt:

bcdedit /set {default} bootmenupolicy legacy

To revert back to the default settings:

bcdedit /set {default} bootmenupolicy standard

Other than the slight delay in startup, there are no known negative side effects to this setting.