Yes, iPhones do have smart lithium-ion batteries that are designed to optimize battery life and performance.
However, letting your iPhone's battery drain to 0% can still have a negative impact on its overall lifespan and performance.
When a lithium-ion battery is discharged completely, it can cause the battery's voltage to drop below a certain threshold, which can lead to a phenomenon called "deep discharge." Deep discharge can cause irreversible damage to the battery's capacity and overall performance.
To help prevent deep discharge and prolong the lifespan of your iPhone's battery, it's generally recommended to avoid letting the battery level drop below 20% and to charge the phone before it reaches 0%. Additionally, it's important to avoid exposing your iPhone to extreme temperatures, which can also affect battery life and performance.
If you frequently find that your iPhone's battery is draining too quickly or experiencing other issues, it may be worth contacting Apple support or visiting an authorized repair center to have the battery checked and potentially replaced.