*For sure they will work on 8.1 if they are released before november! during november probably, by december, its a crap shoot, maybe yes maybe probably leaning more towards no.
*If the game releasing has a physical cart version (not just digital only) and is released between now and during november, it should work on 8.1. Physical games on a cart releasing during december are maybe 50/50
of course game updates are a different story
The absolute best thing to do is NOT to update until everything gets sorted on 9.0.x believe me you will know when games are requiring 9.0 because you will post ... "im on 8.1 and xxxxxxxxxxxx new game is not working, its saying it needs an update" yeah when you start running into those, its time to update. But you should be good at least for the next 8 to 10 weeks for sure.