Lightning Experience Graduates to a Permanent Fixture in Salesforce

Starting with Winter ’20, Lightning Experience is permanently enabled when it’s turned on, either by an admin at your company or via the Lightning Experience Critical Update. After Lightning Experience is turned on, you can’t disable it. All users still have access to Salesforce Classic and can switch between the two interfaces as needed.

Where: This change applies to Group, Professional, Enterprise, Performance, Unlimited, and Developer editions.

How: After Lightning Experience is enabled in your org, the Turn on Lightning Experience toggle in the Lightning Experience Transition Assistant is grayed out.

Disabled "Turn on Lightning Experience" button in the Transition Assistant

And the option to turn off Lightning Experience using the S1DesktopEnabled org preference setting with any version of Metadata API is also disabled.