I can select or deselect UserPermissionsMarketingUser for User detail records manually. The field is visible. However, when I click on the Fields and Relationships for the User object using Object Manager, I don't see the UserPermissionMarketingUser field listed there. Nor do I see it listed in Schema Builder. I checked other user object types found in the following SF link page:https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_userprofile.htm
However it does exist on User Detail records. Verify the following 3 screen scrapes and you will notice they aren't there in Object Manager nor Schema builder but as you can see in the User Detail record that it is there.
How is this possible? See the scrapes below and look for the UserPermissionsMarketingUser field in the listing of the fields starting with "U" (The "M" listing of fields don't include the Marketing User field either). Thanks