If you don't set a volume limit on your iPhone, it will damage your hearing. Apple has a mode to do this, and I've set mine at 80 decibels and rarely have the volume all the way up. The iPhone can output over headphones at 100+.
The smart move is to get headphones that fit well that naturally dampen outside noises and to set your volume limit judiciously.
I am curious if there is data that the volume limit actually does anything to prevent tinnitus over the course of decades of headphone use, especially with active noise cancelation. I imagine enough time simply has not passed yet to know.
The smart move is to get headphones that fit well that naturally dampen outside noises and to set your volume limit judiciously.