The idea would be to introduce an on/off toggle button in AGOL and Portal on a given feature layer that would protect that feature layer's schema from being changed.
The concept here is similar to the 'delete protection' button already available. When enabled, the layer cannot be deleted, until someone specifically turns off the delete protection.
The reasoning here is that, in ArcGIS Pro, it is way too easy to do things like adding and DELETING fields. With more and more users starting to use ArcGIS Pro, already we have had several accidental incidents where feature layers got stuffed up due to a user innocently clicking the wrong thing. It would be great if the content owner/administrator could lock the schema at the server end.