Why is it on Apple to make sure that their private APIs (as in you SHOULDN'T USE THEM, because they will change without regard to your software) are backward compatible?
I get it. I’ve used private APIs in some of my Apple apps. But when you do, you take on a certain level of risk and responsibility when you’ve got customers.
The bigger question is why none of these Electron app developers, particularly larger outfits like Slack, didn’t catch this issue in the last 3 months they had access to Apple OS betas.
Do they have a QA process? Perhaps they should start.
You're right in the purest sense: use a private API, get burnt. But when it's something this widely used and depended upon you could argue Apple should have made it into a public API by now.
With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody. (https://www.hyrumslaw.com/)