iOS 12 Update: Just tested on iOS 12, and it gets out of guided access mode after a reboot.
iOS 11 Update: Just tested on iOS 11, and it stays in guided access mode after a reboot. Same with iOS 11.1.
Update: Just saw an iPad with 10.3.3 not get out of guided access mode after a reboot.
Just a quick observation based on testing:
If you're in Guided Access mode on an iPad running iOS 9.3.5 (say, an older model that can't install iOS 10 and above), and you do a forced reboot (hold home and power buttons until the Apple symbol appears), the device stays in Guided Access mode.
If, however, you're in Guided Access mode on an iPad running iOS 10 (and perhaps in future versions?) and do a forced reboot, the device gets out of Guided Access mode.
P.S. I was able to use this to help a student out who was stuck in Guided Access mode on an older iOS version—updated it to iOS 10, rebooted, and then the iPad was out of Guided Access mode, and a new Guided Access mode passcode could be set.