Microsoft is updating modern list site collection opt-out. They will begin rolling this feature out soon.
How does this affect me?
There is a site collection feature that, when enabled, makes every list in that site collection remain in classic. Today, that feature is hidden and the only way to activate it is PowerShell. We are unhiding that feature so that it is visible on the Site Collection Features page. PowerShell for activating or deactivating this feature will continue to work as it does today. Microsoft will be rolling this out over the next few weeks, and anticipate rollout completion by the end of March 2019.
What do I need to do to prepare for this change?
There is nothing you need to do to prepare for this change. Please click Additional Information to learn more.