Intune

GPO via Registry

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
DWORD: HideNewOutlookToggle
Value: 1

Via 365 Admin Powershell

# Connect to Exchange Online first
Connect-ExchangeOnline

# Disable the new Outlook app for all users:
Get-CASMailbox | Set-CASMailbox -OneWinNativeOutlookEnabled $false

# Or for a single user:
Set-CASMailbox -identity [email protected] -OneWinNativeOutlookEnabled $false

# Verify change
Get-CASMailbox | ForEach-Object {
Write-Host "$($_.DisplayName) - OneWinNativeOutlookEnabled: $($_.OneWinNativeOutlookEnabled)"
}