What problem does this feature solve?

can use the ref sugar to destruct Object

What does the proposed API look like?

const a = reactive([1])
ref: [b] = a; // expect it can be worked
console.log(b) // 1

const c = reactive({d : 1})
ref: { d } = a; // expect it can be worked
console.log(b) // 1

See https://v3.vuejs.org/guide/reactivity-fundamentals.html#destructuring-reactive-state