Relatively easy in what context? It would have to be specified, designed, implemented, tested, and iterated. It’s a fair amount of work.
This thread, which talks about something much simpler, an addon, might give a little insight into why something that exists in a later version might still be a lot of work to backport into an older version:
And there have been (and probably are) addons that can keep track of talent builds, keybinds, gearsets, and whatever else you might want to do, but they won’t change respec fees for you! 