“For Chromebooks released before 2021 and already in use, users and IT admins will have the option to extend automatic updates to 10 years from the platform’s release (after they receive their last automatic update).”
The 10 years count from when the platform launched, not from when a school bought the laptop. A district buying a Chromebook built on a two-year-old platform gets eight years, not ten. Older machines need someone to opt in after their updates already stopped. Google calls this sustainability, and Google alone decides what counts as a platform.