I’m not the author, but the impression that I get is this:
Such an option may have been short-term feasible 2 years ago, when this request was originally posted (and when the art overhaul had just happened, so the game was largely unchanged otherwise).
But at this point, with two years’ worth of story changes now separating the current releases from the version(s) with the original artwork, this is probably no longer possible because the old art doesn’t follow the current story. IOW, there are (newer) scenes where no pre-v10.0 art was ever produced, so it would be impossible to “opt-in” to displaying that content in the old style. Simply because it never existed in the old style.
That’s why wolfstar’s committed to keeping version 9.9 available, so that people can still play the old game with the old-style art if they wish… But it also necessarily (IIUC) means sticking with the old game (plot, story), as it existed right before the art overhaul.