We can consider offer"canAddItems: false" so that the multifield readonly function could be achieved.

Currently we already having "canRemoveItems" configurable, customer would like somehow make it readonly by offering above "canAddItems" function.

Please investigate deeper into the use case and implementation to decide where to go.

Development note:
A read only multi field may also cause issue to a "create" dialog when users want to create a new content node which has a multi field inside.
Also this may work for JCR multi-field but not other datasources. Please consider a general approach there.

Work around:
Use content app to manage that field.