Battery Health at 89% after 11 months

The nominal rating is 500 cycles to a "typical" battery health of 80%. Of course there's no way that every single battery follows that. It's just a couple of round numbers, and 80% is the industry standard for when a lithium-ion battery is "depleted". Some do better and some do worse, as with anything that will eventually wear out. Apple makes no guarantees about the performance other than it will be replaced under warranty or extended warranty if it goes below 80% (before reaching 500 cycles) before the end of the warranty.

What you have might be worse than average, but there's really nothing you can do other than pay for a battery replacement. I understand that currently Apple will do that on demand, when a few years ago they simply wouldn't unless the battery had already dropped below 80%. The battery service cost from Apple is $69 (in the United States) for any notched iPhone. It may be different depending in other countries. The service price may be different from an Apple Authorized Service Provider, but it tends to be about the same price as from Apple. They might require shipping if they have it in stock and they send it to an Apple service center. Also there's mail-in service and that requires the standard shipping fee (just outbound though, and Apple will provide a prepaid shipping box to send it to them). There are other considerations, such as making sure that it's backed up, as Apple or an AASP may delete all data (for various reasons) or even just replace the entire device. Also - Find My iPhone needs to be turned off before any service.

iPhone Battery Replacement - Official Apple Support

If you have any other concerns, it might make sense to carry a USB power bank and short Lightning cable with you, as if can extend the time you can use it. I personally carry an 8000 mAh rated one in my pocket.

Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles”. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

Yes. Most users experience a decrease in health of 1-2 percent per month (at least initially). So many factors affect battery life but the decline in battery power is not linear so you should still have plenty of life left depending on use, environment, etc.