I don't know, my guess would be that the majority of iPhones have their batteries upgraded. Apple currently still gives you money for trading in back to an iPhone 8! They probably upgrade the battery and put it up for sale in the developing world, I would guess.
I only paid $250 for my used iPhone 11, and that's not even as old as they go.
I imagine most of HN is shielded from the flourishing secondary market of old phones because they can easily afford the latest and greatest (counting even a couple years back). But at least where I live in Indiana, there's a pretty thriving ecosystem of yard sales and reuse, and people are not just going to simply throw away a functioning phone. An iPhone that's almost a decade old still has value, and there are repair shops that could put a new battery in it to keep it going for a little while yet.
If you don't think batteries get upgraded, what do you think happens? Do people really just throw their phones in the garbage?
And how many people end up upgrading the battery is yet another quality. I would suspect a small fraction of phones with upgradeable batteries actually gets battery upgrades. Having upgradeable internal components generally correlate strongly with recyclability... however once again, in my pessimistic estimation, only a small percentage of recycling actually amounts to anything.