How to get into Windows 10 Safe Mode on 2016 MacBook Pro?

Basically hold the shift key while booting up or restarting.

This assumes that Windows is your default startup disk. Microsoft changed how to get into Safe Mode on Windows 10. From Microsoft.com:

Restart your PC. When you get to the sign-in screen, hold the Shift key down while you select Power Power icon

Restart. After your PC restarts to the Choose an option screen, select Troubleshoot > Advanced options > Startup Settings > Restart. After your PC restarts, you'll see a list of options. Select 4 or F4 to start your PC in Safe Mode. Or if you'll need to use the Internet, select 5 or F5 for Safe Mode with Networking.

I had a similar problem where I needed to boot into safe mode or access other Windows recovery options, but couldn't because my Mac had the touch bar and for some reason my trackpad and keyboard would not work when booting into Windows, at all. After learning that Windows should automatically boot into recovery mode if there have been 4-5 hard reboots (assuming Bootcamp is your main startup disk), I tried hard resetting (holding the power button down until it shuts down, then pushing the power button to turn it on) several times.

On top of that, considering my trackpad and keyboard would not work when booting into Windows, I instead hard rebooted while holding down the Option button, which brought up the Mac and Bootcamp (Windows) partitions, where my trackpad and keyboard worked. Then, I held down the shift button while I clicked on the Bootcamp (Windows) partition "arrow" button. That booted Windows into recovery mode, which gave me the options I needed and I was able to fix what I needed to.

This may have been a combination of just being lucky (a fluke) with the amount of times I tried hard rebooting and thinking of holding down Shift while clicking on the Windows partition.