Is fast charging bad for battery health?

No, fast charging is safe when you use certain battery chargers with compatible iPhones.

in the link below you can find all the informations about fast charge and compatible iPhones and accessories.

Fast charge your iPhone - Apple Support

Not really, no. This is due to how fast charging works. Fast charging batteries have two phases of charging. The first phase is where they take in as much power as possible. The first phase is usually when the battery capacity is low or empty.

But once the battery capacity reaches that quoted level, the charging speeds are reduced to prevent stress and heat that can damage the battery's longevity. You've probably noticed your phone charges faster to a certain percentage but longer to fill the battery; this is why.

It's also worth keeping in mind that your phone may automatically turn off fast charging if your device's temperature exceeds a certain level.

[Link Edited by Moderator]

As a theoretical matter, anything that increases the heat and stress on the battery will speed up degradation. As a practical matter, will it be significant enough to be noticeable or even affect it more than just random performance differences? You really have no way of knowing, but I'll just say that I feel comfortable charging my battery in any way that Apple approves of. Worst case scenario is that the battery might need to be replaced a little bit sooner.