It's still an excellent idea to power off your phone whenever you are in the vicinity of a border guard and never voluntarily unlock it anywhere close to the border. You can't (generally) be compelled to unlock your phone but you absolutely can have an unlocked phone grabbed out of your hands by a border guard with no legal right to lock it.
I believe it's 100 miles from the border including coastlines but does not include a 100 mile radius around international airports. I don't remember the source but Ive seen a map that represented it that way.
Also worth noting, this ruling only benefits citizens in that specific district, as other districts aren't bound by its rulings. Personally I'd recommend having a 2nd device you can use to record your interactions because if they violate your rights your chances of getting their body cam video of it aren't great.
This is important - power OFF your phone. Your phone is more secure before you unlock it for the first time after booting. Use a strong password as well.
You can also force your device into Lockdown mode, which does the same thing, without needing to shut it down or restart it. It's easy to do quickly once you know how.
On Android it's enabled by default, you just hold the power button and press Lockdown.
Iphones have a way to disable biometrics as well with a button combo, but its more a side effect of activating Emergency SOS, not a dedicated feature and how you activate it varies depending on your device model.