It‘s not. Instead it‘s the only reasonable way to deal with degrading batteries - a limitation all manufacturers have to deal with. Apple opted to prevent unexpected shutdowns by slightly throttling performance - the correct decision for most users, but they didn‘t tell the user about it, which of course serves as a great excuse for colorful, exaggerated headlines like the one in your linked article.
And you can get a replacement battery for $70. I got one for my 6s after two years before the whole batterygate thing started because it wasn’t holding a charge. Between my son and I, we kept it for four years. The only reason I didn’t keep it for another year was because I wanted a larger phone with a larger battery.
I literally linked an article that says they've had to settle for half a billion (!) because Apple did this without informing the users they were doing it. And Apple had an obvious financial incentive to not mention it to users. People found out almost by accident that their phones were still working well, except for a part that is replaceable (and much cheaper than a new phone).
In what world is this not a valid example of planned obsolescence?
Mistake? They made the decision to slow down devices without telling users and profited from that decision.
My iPhone 5 became slower after 1-2 years. If the battery was bad, I never noticed it (no shutdowns or anything like that). It coincided with a new iPhone and iOS version... I tried every trick, from formatting the phone to reducing animations. Then I found a thread on Apple forums about this and someone was suggesting getting a new phone!
I did buy a new phone (OnePlus One) and it's still working after ~5 years. The battery doesn't last as long, but it doesn't die when I receive a call or play a game.
Because of this, that iPhone 5 was my first and last iPhone.
It slowed down the phone a lot, not "slightly". I wasn't asked if I wanted the slow down, there was no way of disabling it, Apple didn't acknowledge it, and I had no reason to believe the battery was in a bad state.
Last year I upgraded from a mid-range phone which was almost 4 years old. It didn't have any "slow down" feature and it never shutdown, so as you can imagine, I'm not willing to give Apple a pass here.