If that's true, by the time that happens the iPhone have already been upgraded. I've use the iPad charger on my 6+ for a whole year and now 6s+ dramatic drop in battery health.
Exactly, and just because someone says something in Apple forums doesn't make it right. Apple officially approves 12 watt iPad charger on all phones and iPads. The phone regulates how much current it accepts based on charge level. As it reaches 99-100% charge only a trickle charge that cycles on and off is going to the battery. So you can leave charger attached for extended times without any damage to phone or battery.
Since the iPhone 6, the phone can take full advantage of the higher wattage chargers. Prior iPhones would only accept up to 5-6 watts even with the 12 watt charger. However I and others had noted faster charging due to the extra capacity being able to run the phone while still providing full wattage to the battery. Remember the phone is still using it's radios, gps, for many functions while asleep.
The absolute fastest charge will happen if you either turn phone completely off while charging or set airplane mode so radios don't broadcast. And use the 12 watt iPad charger.
Apple engineers have made this bullet proof. User can't f this up. Only thing that will harm the Li ion battery is heat. If your phone is getting way hot, some app is hung up, or it's massively downloading resetting first day or so after setup. Beyond that it is possible to receive a defective unit which Apple will replace. Go to Apple Store if heating goes on for days.