Is my battery damaged?
Pretty much.

Smart phone batteries last around two years of normal use. Once the 80% capacity is hit, batteries lose their charge quickly.

Even worse, the voltage decreases which causes unexpected shutdowns. Apple tried to solve low voltage with throttling but never informed the customer. Apple is now hit with plan obsolesce lawsuits.


https://qnovo.com/what-happens-after-80-percent/

https://www.businessinsider.com/how-apple-slows-down-iphones-old-batteries-explained-2017-12
https://support.apple.com/en-us/HT208387