It is rambling because this is about batteries which—surprise—don’t pose a privacy risk. Their stance on privacy doesn’t give them a free pass on anti-consumer behavior unrelated to privacy.
As I said a bit ago, I can usually see Apple’s side in situations like this. I may not always agree with it, but I can see and understand their point of view. But this, yes, is just greed. Completely arbitrary limitation of software functionality for no functional reason. All batteries report the information that Battery Health uses, but Apple is actively choosing not to report it unless they replaced the battery themselves.
There’s no special “algorithm” — it’s the battery’s current charge capacity relative to its original charge capacity, with either a time average or median because the instantaneously reported value will occasionally fluctuate. It’s not about “safety” or a “proper repair” — we can find numerous Apple Store repair horror stories where people left with a device that was objectively worse off than when they brought it in.
It’s greed, and it’s okay to disagree with a corporation even if you happen to agree with some things they do. They’re not holding a gun to your head.