I have heard "string feeling dead" for a long time but I have never understood it. Does it mean "Losing too much tension so that the tension now is below what is acceptable for the user"? Or does it mean something else?
It means the string lost so much tension that the user no longer gets acceptable feedback. Can also be used to describe a string that has little feedback by design (e.g. NBG95).
So does it mean a string feeling dead for one person might be a very alive string for somebody else? i.e one person used to 30 lbs tension vs another who is used to much lower tension.
"Dead" can mean either a) lacking feel (the sense in which I'd use the term), or b) performance has fallen below acceptability through sheer age.