I could see it used for something like that. But it is more general as it can act upon any HTML element. So, you could use it for masking an input element or any other local state based element. Granted, it doesn't need to be limited to local, but at that point HTMX, Datastar, html-form, etc might make more sense.

html-form is the one I made out of those and it is more single focus using the native platform.

So, what use case would you see with ShadowDOM?

I see this more as a way to progressively enhance an element. But in my own use of web components I haven't seen a need for ShadowDOM, but I'm usually just progressively enhancing my page and adding behaviors to my elements. I can see a use case for ShadowDOM when completely creating a new element, but at that point it makes sense to just use a regular web component. Maybe I'm wrong. But that is the reasoning I'm using.