That was an attempt at graceful degradation that wasn’t communicated well.
The general belief was that having a phone that runs a bit more slowly, but consistently used it’s battery as the battery life naturally degrades is better than a phone suddenly dying with 40% battery remaining if the peak power draw exceeds what a several year old lithium ion battery can produce.
I much prefer the former personally, but I get where the misunderstanding and public outcry came from.
Beyond that, what made your phone unusable? The last 4 or 5 iOS versions have improved performance across the board, even for phones several years old.