What problem does this feature solve?

It is not possible to create a watcher that is not influenced by lifecycle hooks of a component if it is created in the setup function. The doWatch method allows passing an instance which would allow passing null and therefore disable lifecycle tracking for the watcher. However, watch/watchEffect doesn't have this parameter. Another possibility would be to create a low-level effect. However, it is not exposed to the Vue-Object when using the browser bundle.

So either exposing effect or allow to pass an instance to watch/watchEffect would solve this issue.

What does the proposed API look like?

Vue.watch(getter, cb, options, instance)
Vue.watchEffect(cb, options, instance)

const effect = Vue.effect

There's an RFC being discussed to allow for a broader solution, i.e. for computeds:

https://github.com/vuejs/rfcs/pull/212

Please give feedback there.