The following article is a reader-friendly expansion of a Popular Mechanics article explaining the basic problem: https://www.popularmechanics.com/flight/airlines/a25923137/hacker-airline-security-flaw/
The article below tho focuses on the regular wide distribution of all details of our lives on social media — including the occasional posting of our boarding passes. Bottom line is don’t do it. But this article (if you want to give them your email and zipcode, explains why.
https://www.travelandleisure.com/travel-news/never-post-boarding-pass-photo