Soft Spun is a manufacturing term for cotton spinning (another is ring spun) - it's trademarked under all kinds of companies, but basically is promoted as a softer cotton.
ETA: Sorry, forgot about the second part of your question. I'm no good at dating T-shirts - there are T-shirt forums online, but soft spun/softspun has been around for a while.
“The illegal we do immediately, the unconstitutional takes a little longer.” - Henry Kissinger
"Do not obey in advance." Timothy Snyder "On Tyranny"
