If a field on the node edit form is a entity relation to a taxonomy term vocabulary, I want the taxonomy terms be restricted on certain conditions. I don't know how to do it safely. I have tried it in
hook_form alter(). Deep in the "array stack" of the form variable is the definition of the entity reference form field.
Is the only way to "hack" the "#process" array, or is there any object-oriented way to filter the items in the drop down element?
Is there any form "plugin" to filter the items in Drupal 8?