Mirrors are an interesting approach to widening the field of view. There have been a number of others, notably anamorphic, which compresses side-to-side, and needs to be decompressed, typically with a reverse anamorphic projection lens, as this was a way to fit a wide view onto an ‘academy’ (4x3) movie frame. More common is simply switching to a wider lens.

FWIW, many photographers are aware of the circular image / rectangular sensor mismatch (particularly anyone who has used lenses designed for a smaller format.

As someone who learned on square (1x1)format, rectangles have always seemed wasteful.