Battery engineer here at an electric vehicle company.
The crux of it is this: when you draw high current from a small battery, the voltage will sag (1). The manufacturer has an on-board battery management system that MUST turn off the battery if it goes below a certain voltage threshold.
As the battery ages, the sag effect increases significantly, up to 2-3x. Thus, when your phone clearly indicates say 20% battery remaining, if a high load were to be applied, the last 20% would be inaccessible because the battery sag from the spontaneous high load would cause a reboot.
Thus, Apple acted in good faith to maximize the battery life on these older devices. The reason is technical, so it's hard to communicate this in a way that all consumers can understand. Yes, they failed at this. Look at all the confusion.
(1) Exercise:
Suppose no-load 3.4V corresponds to 20% remaining.
Suppose that the battery cut-off is set to 3.2V.
The battery % is usually derived from a current sensor (coulomb counting), NOT from the measured voltage.
But the battery cut-off is a safety feature and needs to be measured hard against the voltage (and the current sensor needs to be ignored due to cumulative error).
On a new phone, fully loaded voltage drop = 0.1V
On a used phone, fully loaded voltage drop = 0.3V
Low-load condition, @20% = 3.4V, OK
Full-load condition, @20% = 3.4 - 0.3 = 3.1V, Emergency OFF.