During the first ca. three months of my iPad usage indoors during winter typically at brightness suitable indoors, I charged typically every two days after having used the iPad for 11:50, which I checked every time in the battey control view, where the usage time since last charge was stated, watching the battery running out of juice. Every time, it was very close to 11:50.
(On a few exceptional days, I tested permanent maximum brightness and got 5:30.)
After the infamous minor iOS update, I did the same but now it was close to 10:00 every time. I continued to use the iPad typically only recharging after the battery running out of juice. I continued to watch the battery control view regularly until roughly two (?) years later the view was crippled.
Therefore, I do have sufficient, regular and accurate information on the battery life of my iPad. Besides, during the first ca. 2 1/2 years, the battery duration did not noticably decrease hardwarewise; this only came after 3 years and more from 3 1/2 years on.
One might speculate on what iOS might have done to create the 15.5% lower battery duration after that infamous minor iOS update but what matters is my evidence of aforementioned careful battery status observation with the utmost clear sudden drop from before to after that minor iOS update.
It also fits into the pattern of similar actions of paternalism by Apple, and the timimg is after all the relevant reviews on the Mini 4 were published. If neither purely malicious intent (allegedly forcing people to buy new devices more often) nor a pure bug (or several bugs) without fixing, a third theory is Apple-paternalism of using roughly 80% charge for a longer battery lifetime without ever admitting doing so and without ever giving the user a choice. I would have preferred continued 100% use per charge and expected a shorter battery lifetime.