Yeah, this definitely needs to be added as a feature. It should be incredibly lightweight to perform in their rendering pipeline, but in avoiding your eye going off and letting stuff get out of proportion being able to view the mirror of your document (without impacting it) is huge. Given they already have support for rotations on the viewport, presumably as a matrix, creating a mirror version should be fairly trivial (though, I know, it almost never works out that way once the code hits the compiler).
Anyway - big +1 for this - would be immensely helpful with using Affinity Photo for creating concept art.