The PNR (Passenger Name Record on your boarding pass contains a lot of sensitive and personal information.
NEVER, I repeat, NEVER throw boarding pass (or ay flight-relevant papers) into trash bins
Post boarding pass some days AFTER you returned home to avoid burglery during your trip
Take picture of your boarding pass and REMOVE all flight and personal data, REMOVE also EXIF data
The video Where in the world is Carmen Sandiego from 33C3 (2016) shows what can be done if your boarding pass with PNR data is published in the internet.