Since the introduction of boarding passes (and train tickets), their design hasn’t changed much. This mainly because of the printing technique that is used. It’s not really printing, but colouring letters by locally heating the paper. This also implies that only preprogrammed shapes can be produced, or with more ‘modern’ shapes are build using a pixel grid of about ~5x5mm. Tyler Thompson made a website called Pass Fail that started a very interesting discussion among readers and (graphic) designers. It shows many different approaches and (technical) limits of the boarding pass.