The (urban legend?) I heard was that they changed the formula so that Pyrex couldn't be used for drug manufacturing.

They still handle all temperatures a home chef will ever need, but a drug operation will reach the limits and the glass breaks.

They do not handle the rapid temperature differentials a home chef often encounters when they aren't careful enough. Especially when they are expecting the resilience of borosilicate glass.

Definitely an urban legend. Heat and narcotic substances do not mix very well beyond the barest threshold of baking temperatures.

the new formula was cheaper to make. think thats all you really need to know

Up until a few he Pyrex brand consumer food storage containers seemed to be this other glass (soda lime?, with the bluegreen if you looking into the edge).

Then, a few years ago, they seemed to switch essentially the same SKUs to a third glass (whitish looking into the edge) in slightly different glass forms, and slightly different lid fit). (I don't think they were counterfeit.)

The awful noise they used to make when 2 pieces touched was the brand.

unless you are in the EU, pyrex glass from there is still the good stuff