Don't mean to be disrespectful in any way, but Glock has never made a magazine that wasn't steel-lined. They did change from having the steel lining on three sides to all four sides many years ago. I have Glock mags that are 16-yrs-old, and never had one to fail. Have had Glock rebuild a few for me (new springs and followers). They perform that service free.
Glock must be doing something right. There are over 5M Glocks in use worldwide right now. Over 70% of U.S. l.e. agencies use Glocks. I've fired tens of thousands of rounds through my Glocks and have never had a plastic frame fail.
I have no reason to be making this up. The magazine literally broke in half.