gistfile1.txt

public class MyClass {
    @AuraEnabled
    public Milestone__c milestone { get; set; }
    @AuraEnabled
    public List<Deliverable__c> deliverables { get; set; }

    public MyClass(){}

}

@AuraEnabled
public static void insertMilestoneWithDeliverables2(List<Map<String, Object>> milestones, List<Map<String, Object>> deliverables, List<MyClass> test) {
    List<Map<String, Object>> milestonesWithDeliverables = new List<Map<String, Object>>();

    system.debug(test);
    system.debug(test[0].milestone);
    system.debug(test[0].deliverables);
  }
  
//Javascript
let test = [];

this.milestones.forEach(milestone => {
  let milestoneCopy = { ...milestone };
  milestoneCopy.deliverables = undefined;
  test.push({
      milestone: milestoneCopy,
      deliverables: milestone.deliverables
  });
  }