This was exactly what happened to me. My phone slowed down drastically after an update, and so I took it in. They confirmed to me that it was this update that throttled everything if the battery health was degraded "in order to prevent further battery degradation". But when I pointed out to them that my phone was less than a year old and still under warranty, they tested it again and told me my battery didn't meet the threshold for warranty replacement.

I pointed out the absurdity of their double standard, and resolved to never buy an apple device ever again.