Hello there,
Normally you can find the Focus Assist GPO at: User Configuration > Administrative Templates > Start Menu and Taskbar > Notifications > "Turn off Focus assist" - Enable it to Turn it Off.
Other way would be to propagate a Registry change through GPO to the affected computers. The Registry change should be at:
Path: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Notifications\Settings
Key: NOC_GLOBAL_SETTING_TOASTS_ENABLED
Value: 0 (Disabled)
Last but not least, Focus Assist can be disabled using Powershell (for example included in a Logon Script) running the next cmdlet:
Set-ItemProperty -Path "HKCU:\Software\Microsoft\Windows\CurrentVersion\Notifications\Settings" -Name "NOC_GLOBAL_SETTING_TOASTS_ENABLED" -Value 0
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