If you are annoyed about your users receiving the notification to install Outlook mobile, the announcement MC219490 in the Microsoft365 Message Center in the may be helpful to you!
You can now control if the user is shown a hint for installing Outlook Mobile in either Outlook on the Web or in Desktop Outlook.
For Outlook on the Web we can already utilize an Exchange Online PowerShell cmdlet to control the behavior.
Connect-EXOPSSession
Set-OrganizationConfig -MobileAppEducationEnabled $False
In a later release, this cmdlet above will be able to manage notifications in Outlook for Windows (desktop app) too but for now, you can complete this form to submit your request to block these notifications in Outlook for Windows. You will need to provide your tenant ID. Requests can take up to 7 days to take effect (you will not receive a confirmation that your request has been received or processed).
Cloud Identity - Modern IT 