Hearing isn't a reliable sense of anything on the road. Pedestrians, bicycles, electric vehicles, potholes, low-flying plastic bags, diesel spills, wildlife etc. are all effectively silent, so you still have to look. Deaf people have no problem using the road (although they're generally better at maintaining their awareness of their environment than hearing people who can't hear).
On which basis I'd consider headphones to be Mostly Harmless, and the issue is how much you're distracting yourself with the audio. That's likely to be a sliding scale between 'instrumental music' and 'telephone call'. It's potentially a safer way to receive instructions from a navigation device than a screen on the handlebars.
It's not about the headphones.