The following is not about the Outlook Mobile app but about Outlook Web Access (OWA), also known as Outlook on the Web, when accessed using your mobile web browser.
A new look is available for OWA is now available and turned on by default, but you can opt-out at any time by switching the toggle.
Exchange administrator can manage the visibility of this toggle by updating the OWA policy using PowerShell:
Set-OwaMailboxPolicy “<outlook web access policy>” -OutlookBetaToggleEnabled $false
The new OWA UI on mobile browser looks nicer