On modern SharePoint list and library pages, there´s a link in the lower left of the page says Return to classic SharePoint. This link reverts the current list or library back to classic mode. Also in the classic page, there’s a link that says Exit classic experience. With the last update of the SPO Management Shell you can control these links appear.
- Connect to your tenant with the next cmdlet
$adminUPN="email@example.com" $orgName="domain" $userCredential = Get-Credential -UserName $adminUPN -Message "Type the password." Connect-SPOService -Url https://$orgName-admin.sharepoint.com -Credential $userCredential
2. Remove the links Return to classic SharePoint
Set-SPOTenant -DisableBackToClassic $true
It takes a couple minutes to deployed in the entire SPO Sites, just be patient.
NOTE: this cmdlet comes with the April update of SPO Management Shell, if you need to update to the last version go to this post