I mean it has been in every version.
Actually it wasn't there to start with in Shogun, was added in a patch/bf Mongol Invasion.

I kinda liked it better without retraining because you often got stuck with the choice of merging green troops to an elite but depleted unit at the expense of reducing Honour, or fighting with them depleted at the risk of losing the unit entirely.
Actually I guess that is there in RTW with the Mercenaries since at least in RTR, some of the mercenaries are distinctly useful & rare