bug_firestore.rules

service cloud.firestore {
  match /databases/{database}/documents {
    function isAuthenticated() {
      return request.auth != null;
    }

    match /user/{userID} {
      allow read: if isAuthenticated();
      allow create: if true;
      allow update: if request.writeFields.size() == 1;
    }
   
// It will work on creating new user. 🤔
//     match /user/{userID} {
//       allow read: if isAuthenticated();
//       allow create: if true;
//       allow update: if userID == 'xxxx';
//     }
  }
}

client.swift

Firestore.firestore().collection("user").document().setData(["name": "foo"]) { error in
    if let error = error {
        print("create user error: \(error)")
    }
}