I think this particular issue might not be related to the safe flight envelope, although that was the hot take at the time... I should have added more context.

I think this particular issue might not be related to the safe flight envelope, although that was the hot take at the time... I should have added more context. About a year or so after the article I linked above was published, Bombardier obtained ISA+40C certification for the CRJ700 and CRJ900 from both the FAA and TC. For the flights in and out of Phoenix, AZ, this meant that those aircraft have since been able to legally fly in temps of up to 123.8F. I don’t know the details of the certification upgrade, but my understanding is that there were no changes to either the aircraft or weight restrictions. I don’t personally know anyone at Bombardier to know exactly why they didn’t certify the aircraft for ISA+40C to begin with, but given that there were apparently no changes to either the aircraft or the weight restrictions, this seemed very much a case of “your engineering team had to wait ... to realize an item that's going to be outdoors in the sun is going to get hot”.