does nightstand mode kill battery health

If you have charging optimization turned on, your phone will only charge to 100% before a normal time that you would take it off the charger. This is to prevent battery damage and the phone will "learn" when your charging habits and adjust to them. Nightstand mode should not cause an overcharge of your battery. The phone will keep it at 80% until a little before you would normally take it off the stand so that it can finish charging the battery right as you need it.

I have read that it is not good to charge your iPhone 15 Pro Max consistently to 100%

And you can also read that you can connect the phone to the power supply and leave it connected all night....every night.....if you enable Optimized Battery Charging and let the phone charge to 80%....and then very slowly trickle charge over the next 4-5 hours up to 100%......so you start each day with the battery at 100%....not 80%.

This provides better battery performance for those of us who need the phone to go all day on a charge.

Even if you enable the 80% limit option, the battery will still charge up to 100% "occasionally", according to Apple. When I've tried this, "occasionally" appears to be about every 7-10 days, but I'm sure that "occasionally" means something else for another user.

My opinion.....the 80% Limit option was added as a gimmick because there are articles on the Internet that claim the battery will explode if it is charged more than 80%. They never mention that it is impossible to overcharge the battery in an iPhone. So, there goes the idea about never letting the battery fully charge.

I've always charged the battery using Optimized Battery Charging for many years and typically get 4-5 years use out of a battery. If the 80% limit might allow another month or two for a battery, it's not worth it for me, since I want to start each day with the battery fully charged.

But, some users seem to be happy with 80% of the performance when they paid for 100%.