I feel the most common use case of attachTo is still attachToDocument, since that is necessary for testing any Vue code that utilizes document.querySelector and similar methods, even if run in total isolation. By un-deprecating attachToDocument, we prevent developers who are new to the library from having to hunt for the following code.

    const elem = document.createElement('div')
    if (document.body) {
      document.body.appendChild(elem)
    }
   wrapper = mount(Component, {
    attachTo: elem
   })