The truth is more complicated. First of all, only two things really degrade your battery apart from time itself: heat and voltage. Ideally, to extend the battery's lifespan you should keep it at ambient temperature (not exceeding 35°C) and around 3.8V all the time.
If you have a device with lets say 15% battery left at around 3.6V.
Slow charging: it will gradually ramp up the battery's voltage and produce little heat over the couple of hours it takes to charge it until it reaches its max at around 4.35V.
Fast charging: it will immediately ramp up the battery's voltage as it reaches 4.35V and produce a lot of heat at first (if the device doesn't thermal throttle the charging itself which happens at around 35–40°C) and then gradually decrease the speed until it will charge as slowly as with a slower charger.
In reality, fast charging lets the battery stay at around 4.35V for longer which usually means like 20–40 minutes up until slow and fast charging have the same speed. Compared to leaving your device plugged in at 100% over night (meaning 4.35V for hours) the impact is negligible. The one thing that does make a huge difference is heat.
The heat output of fast charging can be quite significant. Slowly charging can mean around 30°C while fast charging pushes the battery to a constant 35–40°C. But unless you are in a really hot environment, it will not stay that hot and if it does, it will decrease the charging speed which reduces the voltage and the heat output but also making fast charging not fast anymore.
It all depends on the safe guards manufacturers implement. If you have a phone with 180W fast charging that allows the battery to be at 4.5V until 45°C it is much more damaging. Yet, if the safe guards are activated too quickly, fast charging only works at certain scenarios like cool room temperatures. Fast charging probably is less hurtful to the battery than leaving your device at 100%. The dose makes the poison. Do you want a battery that lasts for 10 years but at 40% capacity or one that lasts a months at 2x capacity? [I'm just exaggerating here, not actual numbers or even safely possible]
Apple's Viewpoint
Apple's viewpoint probably is that faster charging is not a priority if users are primarily charging at night while the battery lasts a full day. Fast charging could mean batteries having less than 80% capacity within 2 years making it eligible for a warranty claim and lastly, if fast charging is throttled due to thermals, people would complain that it doesn't work. I'm guessing if at all, Apple will enable it on beta builds to evaluate. Probably also a reason they improved the cooling system.
Those new A18 chips are based on TSMC's N3E with higher yields and more efficiency compared to the A17 N3B. So the new chips should run cooler than before yet Apple improves the heat dissipation. Their new cooling design might very well be part of keeping the battery cool for improved fast charging. Killing two birds with one stone: keeping the battery cool while keeping the SoC even cooler.
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