Is it safe to use a Pixel 4a charger for an iPhone 15?

I guess it’s fine, but why would you use that? Why not use the cable that came with your iPhone that’s designed for it? You may experience varying charging speeds and data transfer speeds.

I suggest that you double-check that your Pixel "charger" is Mfi-certified. If it is, it should be ok to use with your iPhone.

Ref: Mfi Licensed Accessories - Apple

if it does not output higher volts than 5volt

and does not send big spikes in the volts every now and then it can never harm

i pretty much only use my oculus quest 2 sub-c charger to charge my ipad m2