Isn't the most obvious end game just (if using the same packaging) some note on a spec sheet of "12 hours screen on time (10 hours in the EU)"?

If it's not configurable people will likely complain battery life is higher on the US's software version, they won't care about the reason.

Well, Apple was already fined for decreasing the CPU frequency (to avoid spikes on aged batteries), so that's not really an option. (Even though at the time they wasn't doing it out of malice at all, they actually tried to keep old phones usable - their marketing team messed up there big time)

The easiest is to just require it be replaced under warranty - if the battery has to be usable to 1000 cycles, and it is at 80% and 999 cycles but doesn't "work" it's a warranty replacement.

But that then brings in a "how many years" question.