I want to know if people will get their Pokemon back or if they'll just stay trapped there forever in XY, ORAS, SM, & USUM
If you have a Pokemon awaiting trade on the GTS and you haven't either taken it out or received the Pokemon it was traded for as of the shutdown time, it will be lost forever. Even if Pretendo ever gets their XY-compatible servers up, they will be a different set of servers with no knowledge of what Pokemon were deposited on the old GTS.