On modern SharePoint list and library pages, there’s a link in the lower left of the page that says “Return to classic SharePoint.”  This link reverts the current list or library back to classic mode. On the classic page, there’s a corresponding link that says, “Exit classic experience.” 

By default, these links are shown on all list and library pages to all users. Starting from May 2021, tenant administrators can now control whether these links appear.

To remove the links, run the following PowerShell cmdlet:

Set-SPOTenant -DisableBackToClassic $true

To show these links again, run the following PowerShell cmdlet:

Set-SPOTenant -DisableBackToClassic $false

So basically, the Set-SPOTenant command has been updated to contain DisableBackToClassic parameter.