So you know i just purchased a iPhone 8 brand new from apple updated the device to new iOS and my iPhone is already at 94% battery life. apple put this feature in because of there lawsuit a tried and true way to see if you have a bad battery is to check a few things.
1. is the battery swollen, lithium ion batteries do have life spans and will go bad just like your AA batteries swelling is a sign the battery is starting to go.
2. does the phone slow down and get laggy around 20% battery life, thats the phone attempting to slow down to keep the phone battery lasting longer its a sign battery could be replaced
3. the most annoying one do you charge your phone to 100% unplug it and it drop to something like 80% in a short time, thats a large sign that battery needs to be replaced
Lastly i would say that i wouldn't listen or worry to much about apples new battery life tracker its also just a beta model and apple is never good at being totally upfront about things so just because it says 80 or 0 if the phones working i wouldn't pay apple 100$ to replace it or replace it yourself with repair guide on ifixit