Most of the dev conferences around the Midwest have both the QR ticket code and then you can print a physical ticket as well - just in case. Speaking with people who work at the events, physical tickets are a life saver in conference halls where wifi/cell coverage is awful.

Nobody volunteering wants to be the bad guy and tell someone they can't enter the event because they don't have a digital ticket.

Of course a dev conference can be lenient. A Red Hot Chili Peppers concert? Probably not so much.