When said app authors use a private library that a Apple asked them to not use, is it really wrong to blame them?

From a Windows end-user perspective, the entity to blame for apps breaking after a Windows upgrade is Microsoft, even if the app was doing something it should not have [0].

[0] Search for "return policy" on https://www.joelonsoftware.com/2004/06/13/how-microsoft-lost... .

This was clearly a vendor bug though.

Apple is a fashion brand, and Apple users buy it for that reason, not because of any technical merit.