In The Search for Spock, the new USS Excelsior is equipped with a "transwarp" drive. The usage of "transwarp" is different here than in Voyager, as explained in
In particular, "transwarp" in The Search for Spock refers to the ability of a ship to jump instantaneously from a standstill to any particular warp factor in the normal range of warp factors (although not every source agrees on this particular interpretation), whereas the Voyager references to transwarp, particularly in "Threshold", involve a warp drive capable of breaking the Warp 10 barrier. (On top of this, there are the Borg transwarp conduits.)
Concerning strictly the particular type of transwarp represented by the transwarp drive on the Excelsior, my question is:
What is the fate of the transwarp experiment on the Excelsior? Of course, in The Search for Spock, we only see the results of Scotty's sabotage ("from one surgeon to another..."). Presumably, the transwarp drive worked, at least in theory, and it was likely tested in simulations and in reality on smaller craft before being fitted to the Excelsior.
When we see the Excelsior again in The Undiscovered Country, it doesn't appear to have special warp capabilities — at least if it does, then no one seems to be bragging about it any more.
Did the Excelsior's transwarp drive eventually become standard issue for Starfleet ships, and henceforth just referred to by the usual moniker "warp drive"? Or was it shelved and, if so, why?