I thought Turtles Forever made it known, and various references to it have been made by the new show. Hell I'd argue fans of the 80's show already know about it by now.
Even so, Mirage is still largely an outlier, even among those who would consider themselves "TMNT fans".
Again, the most I'm hoping for is that this game will have plenty of alternate costumes/skins that can pay tribute to the various eras/iterations of the TMNT. Even better if it includes a "black & white" mode, so it could go with the Mirage-era skins that much better.