There are three ways to get Windows 10 into TROUBLSHOOTING / SAFE MODE without logging in:
- At the login screen click the round power button at the bottom right corner of the screen, press and HOLD the SHIFT key and click RESTART
- Insert a Windows 10 DVD and boot off of it, then click REPAIR YOUR PC from the bottom right of the first screen with prompts on it.
- Crash the PC. If Windows detects that is has crashed before completing its boot up, it will automatically enter TROUBLSHOOTING / SAFE MODE on the next power up
Microsoft removed the old press F8 on boot logic because it slowed boot up times and ever since it has been a pain in the butt to get Windows into Safe Mode (now named Troubleshooting).