I have read most of the posts here and the repeated points on battery endurance and some fluffy soft wording from some that seem to be more from Apple legal team
Factual:
There is/was a design oversight relationship between CPU demands and battery capability over time on some models
An update was introduce that fixed battery endurance to the detriment of CPU performance over time
Apple did not disclose the side affects to the CPU when improving battery endurance over time
Apple standard warranty varies between 1 or 2 years depending on country of origin and can be extended with AC/AC+
Apple only replace batteries that are in warranty based on their own testing procedures/policy
Apple have clauses in their specifications and sales T&C's to cover normal battery degradation over time (Typical for all OEM's)
Apple have clauses in their software T&C's that place burden of acceptance on the owner with out the option to revert and only opt in or out with little clarification
Some phones will stay on longer but at a slower CPU speed with the fix/update
Apple have clauses in their T&C's for usage of their products to avoid claims in (EG life threatening or emergency situations)
Argumentative conjecture hear say etc:
Apple was aware of design oversight from the start
Apple deliberately obscured the side affects of the battery endurance improvements
Apple in house battery test is limited
Apple introduce the fix to their advantage to avoid warranty claims, over burdening support services and to miss lead consumers of actual status of their phones
Apple introduce a fix that breaks original terms of sale re performance over time
Apple wording in T&C's, specs, releases etc etc is deliberately lacking some specifics
Apple chose the best solution for the fix for the consumer
etc etc and any other derivative
The real shame with this regardless of the outcome will be a few $'s more on your next purchase as shear numbers will win out in the end.
I certainly hope that on future models if they include this fix/update include a clause that "your performance may vary over time and is dependent on battery condition" so consumers are aware regardless if there is a note or pop-up on your phone