If you upload an avatar with an EditorOnly object, that object is still uploaded with it. The attached screenshots show that this sphere child object is EditorOnly in Unity, but still shows up in game. Even tagging the VRC_Avatar Descriptor object itself as EditorOnly will have the same result.
1. Make an empty gameobject and give it the VRC_Avatar Descriptor component.
2. Give it a child object and change its tag to EditorOnly.
3. Upload the avatar.
4. Use it in game and see that the object itself is still there.