As you know, you can register applications on Azure AD when your application need to have access to data hosted on Azure AD (groups membership, users profile details…).
Well, you can now recover a deleted registered application up to 30 days after deletion.
This can be useful when an Azure AD administrator and/or application administrator/developer delete an application by mistake.
To recover (and list) delete Azure AD application, connect to your Azure AD portal (https://aad.portal.azure.com/) and access the App registrations blade (or Azure Active Directory\App registrations)
You will see a new tab called Deleted applications
You can then see the list of deleted application (including when it was deleted but no who did it) and select the one(s) you want to recover using the Restore app registration. You can also ‘clean up’ this recycle bin by selecting the application(s) and choose Delete permanently.