Don't forget iOS 26.6 also includes over 75 security fixes. After you update, you can expect the battery life to be impacted for about 4 days while the file indexing is being performed in the background and also getting your device ready for the next major release of iOS 27. While this is being done, there is also a confirmation message in the Battery Settings letting you know that.
For me, there has not been any change in my battery life after the setup has been completed. As always the Battery Health is the biggest factor on your battery life, and once it drops below 80% it will need to be replaced no matter which iOS version you are using.