I think the battery will continue to be considered an exhaustible part (it might fail, but usually is replaced because it is simply worn out through use), and Apple may continue to have spares for replacement, but might not procure new batteries. So, I would expect that if a service shop has a battery on the shelf, and it's the one you need, you can get it replaced.
And, new batteries should be available from third-party suppliers, like batteries for most laptops that are discontinued.
Other kinds of parts, like keyboards or case parts, will mean searching, and won't be a normal service option at an Apple shop.