The "visible in masks" option has been removed, but we are thinking about bringing it back in V7 right now.
Currently the solution is to either:
- use the existing mask propagation options (if possible)
- render the scene normally, with the object visible; then enable the "render only masks" option (Render Setup > Scene), hide the object, and render the masks without the object
- or revert to Corona 5
Can you also explain why exactly you would like to hide the object in masks? Is it because you want to render masks of objects which are behind the glass object?