I'd assume their server shutdowns is based on how popular said servers / games are.
If a game has a low player count, then it's probably not worth them keeping those servers up.
On the other hand, NHL Legacy probably still remains one of the more popular NHL games, especially given how below par the NHL franchise has been this generation.
Just purely off speculation here, but I don't think NHL Legacy Servers are in any risk of being shutdown anytime soon.