The IPhone 11 was discontinued two years ago on Sept 2022 and who knows how much the phone was used. Your battery is probably at the end of its life and needs to be replaced.
Batteries degrade over time with usage due to chemical aging, that's just how they work.
You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.
Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.
iPhone Battery Replacement.
iPhone Battery Replacement - Apple Support
About Lithium Ion Batteries
iPhone battery and performance - Apple Support (IN)