Originally Posted by
eigenvector
It shouldn't be shared publicly. As you note, the booking reference plus the passenger's name generally lets you make changes to bookings. I don't see the problem with emailing it to a friend or relative, though.
+1
Go in Instagram or Twitter and look up hashtags for travel, airplane, ticket, boarding pass and the like and you will find hundreds of people posting pictures of their boarding passes. There is nothing stopping someone malicious from ordering them kosher meals, moving their seats, or outright cancelling their flight.