It is a security measure for your protection. Otherwise, anyone can reboot your phone and have it unlocked. I don't know where you got the idea that this is a recent implementation, as it's been that way for many years. After a reboot, a missed call or missed notification, your phone requires credentials to unlock. If you never want to enter your credentials, then the only way is to eliminate all security.
This really annoys me.. why isn't just a fingerprint good enough? Cell phone companies act like we're all living in a mob movie and have to worry about someone hacking off our fingers so they can access our phones. So **bleep**' stupid!!
You can set up an app to automatically display the login screen, input the pin automatically and lock the phone again. If you set it to launch at night every 3 days while you sleep, you'll never even realize it and android won't bother you with this ever again. One such app is called macrodroid but there may be others.
