My iPhone battery health is 89%

Your battery is considered a consumable part. Over time, its performance and health will decrease and this is not only normal, but expected. How often you charge your device and how much you use will determine how fast this happens. Apple considers a battery to be functioning normally until it reaches 80% of its original maximum capacity. So right now, all you can do is adjust your usage to help manage the battery health.

https://support.apple.com/en-us/HT208387

Your battery health gauge is being recalibrated, so it may not actually be 89%; it might be more or less. Wait until the recalibration completes to see its actual value.