Allow Site Admins to Disable "Return to Classic Sharepoint" Link
Displaying the "Return to Classic Sharepoint" and "Exit Classic Experience" links, with no option to disable these links, is a terrible idea. When a user switches from the new, modern interface to classic, it allows them to view links that they are not otherwise permitted to view, thereby overriding permissions. I don't want our users to have ability to switch between the two experiences.
Ron Verma commented
Totally Agree on this but have a different perspective of this problem. Based on my talk with the Support person, when a user hit "Return to classic view" from the modern library view, this sets up a cookie in the browser. Now under that same browser session, if the user goes to some different doc lib (that has the modern view settings selected in the library settings) under some different site collection then that library will also have the classic view when the user lands into that library. This happens coz there is a cookie setup at the browser level that overrides the settings of the document library. So unwantedly, it effects other document library experience also. If the cookie has been setup based on the document library URL somehow then this issue would not have been there. Really want this issue to be fixed.