I have thread left to me by my mother who died in 1986...and she had thread left over from her mother, who died in 1964.
The sewing machine shop said to toss it because it isn't any good.
I have put it aside, but I still choose to use it on occasion. For the extra linty stuff, I just clean the machine more frequently.
I am also saving any empty wooden spools....for a project I have yet to discover!