Masks work the same way they worked before 2.8

Masks are not edited in the compositor: Create, manipulate and animate your masks in the image editor or the movie clip editor. You can add keyframes to the spline points.

To use the mask in the compositor you use an Input > Mask node.

[![enter image description here][1]][1]

There is no need to render masks as images.

  [1]: https://i.sstatic.net/hJHxf.jpg