The metaphor of "glass" is intentional. It highlights three toxic dynamics:
In the world of avant-garde fashion and costume design, the concept of a "glass collar" does exist, often inspired by the Cinderella aesthetic. cinderellas glass collar
Cinderella in the tale is praised for her meekness and her gratitude. She thanks her stepmother for the chance to clean. She thanks her prince for noticing her. She never demands the slipper; it is bestowed. The metaphor of "glass" is intentional