Apple is consistent in their warnings to not use private APIs, and especially don't override them for custom implementations which is what Electron does here.

The _cornerMask override was a hack that shouldn't ever have existed in the first place, and it's not the only use of private APIs in the electron code base.

Apple is very clear about how they want you to make software for their OSes. It's 100% on electron that they choose to do it this way regardless.

I'd go as far as to say Electron itself is a hack that shouldn't exist, but sadly everyone has decided it's the only way they are going to make desktop software now.