Late to the party, but just stopped into the Apple Eastview (Rochester, NY) store while here on business trip to get an iPhone 13 Pro battery replaced under AppleCare+.
Running 18.7.8 so it is a supported version but the nice person who helped me explained they typically require iOS to be at the most recent version for battery related checks to be performed once the battery is replaced.
I noted for Apple to not post somewhere when scheduling service that your iOS will be upgraded to the most current version as part of a battery replacement is not very customer facing (wasted about 1.5 hours of my time and bit of there time).
Will wait until iOS 27 comes out (or a future version of iOS 26) as our org has had poor luck with iOS 26 on multiple models.