The reason for throttling after battery health dips below a certain point is to prevent the SoC from creating spikes in power demand that the battery is unable to service (which becomes progressively more likely as the battery degrades) so the phone doesn’t crash at inopportune times.

Back in the mid-2010s when I was broke and getting by on a worn out hand-me-down iPhone I would’ve loved to have that feature. It really sucks when your phone dies in the middle of a business call at 40% battery thanks to one of those aforementioned spikes.

Any idea why it only happens to iPhones? I don't think I've noticed this on Androids, where the only impact of longevity is the lack of updates from the manufacturers and then Google Play Services starts to fill up the storage with updates.

It does happen to androids (at least my old Xperia xz2)

It does happen to Androids. Its just that by the time it matters, the manufacturer has abandoned it.

aha that makes sense!