While renovating a bathroom, during demo I discovered that the pipe leading from my water valve up to the shower head had a slight bend in it, causing it to press against the inside of the greenboard that used to be installed over it. Over the course of 15 years, vibrations in this showerhead pipe caused it to develop a flat spot where it had rubbed against the greenboard.
I have since bent the pipe inwards a little, at that location, to stop this abrasion. And, I believe it is safe to not replace, since this pipe never sees much pressure (as the shower head is never in a sealed state.)
But, I thought it wise to at least ask for a semi-professional perspective on the topic.
I would estimate that 1mm or less of thickness has been lost.
If I have to replace it, then I'd need to call in an actual plumber to sweat it out, and a new one in.