End-users accessing their mailbox on Exchange Online with Outlook on the Web (aka Outlook Web Access) will start seeing a toggle to invite them to try the new Outlook on the Web interface.
The toggle will be available on the top right
As administrator you may want to disable this capability; to do so you need to update your OWA mailbox policy to turn off this capability using the following command (NOTE this is not available through the OWA mailbox policy UI on Exchange Control Panel)
Set-OwaMailboxPolicy <your OWA mailbox policy name> -OutlookBetaToggleEnabled $false
You can also apply this to all of your OWA mailbox policy at once with the following command
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy –OutlookBetaToggleEnabled $false