Generally speaking, when a battery gets to ~80% of design capacity, it's pretty much at the end of it's most useful lifespan. Sure, it still works well below 80%, but that's roughly the threshold you start to see lifespan degradation and, as we learnt, that's when Apple starts throttling performance.
If your iPhone is on iOS 11.3, go to Settings/Battery/Battery Health. If the Peak Performance Capability says "Your battery is currently supporting normal peak performance.", then your still good.