[2020] Does anyone know why there is a unused Nea Shirt?

It's because it's not actually unused.
http://steamcommunity.com/sharedfiles/filedetails/?id=1188209247

It's a dev/mod exclusive. Or a cheater exclusive, until they get banned for cheating.

Originally posted by Mojo: Dirk Me Gently:

Originally posted by Cetren: The devs use it.

This.

I believe on the back it has the word "Crew"

Correct.