See Reminders to Transition to Lightning Experience
Where: This change applies to Salesforce Classic and Lightning Experience in the Group, Developer, Professional, Enterprise, Performance, and Unlimited editions.
When: The reminders start appearing on March 18, 2019.
Who: Only admins with the Modify All Data and Customize Application user permissions see the reminders.
Why: The first reminder, “Lightning Experience Turns on Soon,” provides a countdown to the auto-activation date of the related Turn on Lightning Experience critical update. It appears in Salesforce Classic every 45 days. It stops appearing when Lightning Experience is turned on or after the update starts to activate on October 12, 2019.
The second reminder, “Meet the Transition Assistant,” encourages admins to check out the new Lightning Experience Transition Assistant, which is the central hub for steps and resources. The reminder appears once a week until the admin clicks the button to visit the Lightning Experience Transition Assistant.
How: You can turn off the reminders. From Setup, enter User Interface in the Quick Find box, and then select User Interface. To stop showing a reminder to admins, select Disable the “Lightning Experience Turns On Soon” admin reminder or Disable the “Meet the Transition Assistant” admin reminder in Salesforce Classic.