AppleCare+ does cover the full cost of battery replacement, but only for 2 years. I don't know what threshold they set for replacement, but IIRC it's 80% of max capacity for their laptops.
I don't know how these new headphones perform, but I would doubt that many people would get below 80% in two years (even with increased usage due to COVID).
It might still be worth it to get the warranty though, since it covers accidental damage (with a deductible), and the cost for the warranty is the same as the cost for battery replacement ($80).
It's a big worry on the smaller AirPods, since those are almost 100% battery and glued together (and I'm not sure how you could build a product like that in a way that's both small and has a replaceable battery). For the big cans, you can pay $80USD to get the batteries replaced if the headphones are out of warranty (and, presumably, the warranty will cover the full cost of a battery replacement).