It only kicked in under certain conditions so I’d bet it was based on monitoring current battery output. I had a phone affected by it and first noticed the problem in the winter when cold temperatures meant it was relatively easy to trigger outdoors but it still never happened indoors.

It's based on battery performance. So technically correct, they just didn't inform users about it which in the end had the exact same effect as if Apple deliberately slowed down phones in order to raise sales. Effectively it was planned obsolescence with (more or less) plausible deniability.

if battery voltage < 80% then slowdown y%

As I understand it, it kicked in after the first brownout.