Battery health

Typically no, but if you own iPhone 12, a recent update did specifically adjust the battery health of iPhone 12 to be more accurate. For some people immediately after that update, they did see a reduction in battery health. But it didn't actually reduce battery health. It adjusted battery health to be correct on that device.

To be specific an update cannot affect battery health.

Again, updating iOS can't impact Battery Health. But your battery health reading of 100% may not actually be real and updating may bring your battery health measurement to where it actually should be. Battery health declines whether you like it or not. It is a measurement based on the number of full-cycle battery charges. And batteries decline period. ALL BATTERIES. You may well see the battery health measurement lower after updating. But you are also likely seeing an inaccurate battery health measurement now.

As I said above, updates don't reduce battery health. Which would you prefer? An accurate measurement of Battery Health on your iPhone or one which is not accurate, but gives you a number you like better? I'd rather know the real measurement as opposed to a false one.

Oh it may. Just not accurately, which is why Apple released an update specifically for iPhone 12 phones to report Battery Health accurately. You don't have to update, but you'd be deluded into thinking the battery health on your phone isn't declining, which is actually impossible.

I just don't understand why people here are so hung up on a number. The number is only an approximation of the battery's health. It is not exact, but I'd want to have the closest to reality on my phone. And your battery health will continue to decline until such time it reaches 80% or less at which time it will be time to have the battery replaced.