Spock stated that Scotty-Prime created the Transwarp beaming equation. Since he didn't do this pre-Jenolin then it must have been after the TNG films but before Ambassador Spock went back in time.
Therefore the Prime universe will invent Transwarp beaming around 2379-2387. Whether it was ever used or Spock was simply called in by Scotty to look over the finished equation is another matter.
The Scotty cliffhanger at the end of Indistinguishable From Magic is intended to be read as him testing out that potentiality (ISTR putting a line in, echoing the "if space itself is moving" from the movie)