I'm not saying they won't trade him; I think they should, but asking for a players no trade list is pretty standard, just because they asked for his 15 teams doesn't mean Trouba will be traded. I do firmly believe he is very tradable though with only 2 years left on his contract.
Depending on the return, would the Rangers have to take a contract back, I would guess the Rangers would have to retain some of his cap hit though, maybe 2 or 3 million. As a $5M or $6M player Trouba will absolutely have value to some teams. Even if the Rangers had to retain $3M, the $5M they would free up would be huge to add a RW and a bigger dman that fits better to play with Fox and would allow them to move Lindgren down.