If there was an option that was presented to users once the device got below 80% battery health to slow down the system to make daily batter life longer
This isn’t why they did it. Degraded Li-ion batteries cannot sustain their rated voltage at high currents due to increased internal resistance. Sufficiently undervolted CPUs/memory cells produce errors (specifically bit flips), which can rather quickly lead to memory corruption and a crash.
Reducing the CPU frequency (thereby reducing the peak current draw) is practically necessary in the face of a degraded battery. Various laptops were infamous for not doing this, because it resulted in a ~20-30 minute battery life, as the voltage drop became too great once the battery charge drops below 80-90%. Within the context of a smartphone, neglecting to use the remaining 80-90% would make it basically useless.
What Apple (and the rest of the smartphone industry, at this point) really needs to do is make their batteries replaceable.