Battery replacement program
Do all iPhone 6 quality for the battery replacement program?
iPhone 6
Question marked as
Yes. See the following from iPhone Battery & Power - Official Apple Support
Through December 31, 2018, the out-of-warranty battery service fee is $29 for all eligible iPhone 6 or later models. Battery service at $29 may be limited to one repair per iPhone. After December 31, 2018, the fee will change to $49 for all these products except iPhone X, which will change to $69.
There aren't any other eligibility requirements in that article other than:
If your iPhone has any damage that impairs the replacement of the battery, such as a cracked screen, that issue will need to be resolved prior to the battery replacement. In some cases, there may be a cost associated with the repair.