An internal memo at Apple has indicated that battery replacements for all eligible devices for the replacement program are now in stock and readily available. This means that if you are wanting to get your battery replaced in your iPhone, it should be something that can be done when you make a Genius appointment.
When Apple announced (or let it slip) that they were throttling iPhones with older batteries, the outrage was pretty quick and loud. In response, Apple began offering discounted battery replacements to customers who had batteries that were bumping into the throttling software inside of iOS 11. Normally that battery replacement is $79 but through the rest of this year, it is $29.