Under a year with 89% maximum battery capacity.

No, that's about average.

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.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone Battery and Performance - Apple Support (IN)

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple

Perfectly normal. Expect Mac Capacity to decline an average of about 1% a month over time. That's an average though.

Some months the Max Cap may not decline at all, then might drop 2% the next month, etc.

So, let's say that your iPhone is 11 months old. It should at about 89% at this time. Right on schedule.