If you are not able to delete a "Native" Azure AD registered application because of inactive "delete" button in the Azure Portal, there is a solution.
The problem (red arrow):
The solution (green arrow):
Just edit the app's manifest and change
"availableToOtherTenants": true to
The issue is that there is no "Multi-tenanted" switch in the "Native" application's blade in Azure Portal. "Web App / API" application has it: