Depends on how you define it. I used “needs replacing” to mean “the average user will feel the battery is not meeting their needs”, which I believe 80% is generally accurate. This can be demonstrated by apple care allowing for battery replacement after the device is below 80% battery health.

If you are using “needs replacing” to mean “the device will be non-functional or risk permanent damage”, you can go significantly lower than 80%. In fact, I think most devices could go down to 0% battery health unless the battery swells.