Poor iPhone battery life

This behavior is expected. All rechargeable batteries are consumable components that become less effective as they chemically age. As lithium-ion batteries chemically age, the amount of charge they can hold diminishes, resulting in shorter amounts of time before a device needs to be recharged. The Battery Health screen you're referring to includes information on maximum battery capacity and peak performance capability.

Maximum battery capacity measures the device battery capacity relative to when it was new. A battery will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges. A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. Note that maximum capacity does not degrade linearly with cycle count. For example, your maximum capacity might be "stuck" at XX% for a while. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge. Learn more about charge cycles, or read a more in-depth guide into batteries.