safe mode key F8 (or shift-F8) is not working, becauase booting process is too fast in Windows 10.

To make it work (despite a little delay during booting),
Command-X -> Command-prompt (Admin) -> input as follows:

bcdedit /set {default} bootmenupolicy legacy

(to recover, set stardard instead of legacy.)

NOTE:
You have to press fn+ F8 key on apple keyboards.
NOTE:
If you keep pressing F8 key after switching on, it is not working.
Just type repeatedly F8 key to make it work, while pressing fn key.


cf. It is said that the following command is also working.

bcdedit /set {bootmgr} displaybootmenu yes

The default delay time is 30 seconds. To change it input as follows (ex. 5 seconds):
bcdedit /set {bootmgr} timeout 5

But after changing to the legacy policy, the booting time is still fast enough.