After updating Windows 10 to Insider Build 17063, I need to run the following command on the Ubuntu WSL (Windows Subsystem for Linux) so that my SSH command can work again, where previously there was always a warning "WARNING: UNPROTECTED PRIVATE KEY FILE!".
The solution is to simply run this command on Ubuntu WSL, and it's done :).
sudo umount / mnt / c
sudo mount -t drvfs C: / mnt / c -o metadata