If you are wearing a suit, a smartphone makes the trouser pockets bulge a lot, so it's common to keep it in your shirt pocket or jacket inside pocket - Both of which is next to your heart.
The FDA [1] advises "don’t carry the phone in a shirt or jacket pocket directly over the pacemaker." as well as the American Heart Association [2] "Avoid keeping your cell phone in your front chest pocket."
One hopes people with a device keeping them alive would have an incentive to follow instructions, especially if it doesn't involve addictive substances. They also get told to avoid metal detectors, and they know to do this, so it's probably, hopefully, an easy habit to change
I do. I don't leave it there generally, but its often easier to get my phone in and out of my shirt pocket when I am seated, crouched, or otherwise actively engaged with something and I don't want to stand up.
It's the most convenient place for a phone on your person.
Easier than pants pockets if you're sitting down. No fumbling with inside jacket pockets. And you can use your "normal" jacket pockets to keep your hands warm without worrying about your phone falling out.
If you are wearing a suit, a smartphone makes the trouser pockets bulge a lot, so it's common to keep it in your shirt pocket or jacket inside pocket - Both of which is next to your heart.