For PvP it shouldn’t really matter, but if the game works around the whole theme of a 1.9 specific item, then it’s going to have issues. For example Layer Spleef and Paintball etc would be fine as they don’t have any 1.9 specific features other than the glowing effect, which may impact the game slightly. But games like Wing Rush will have major issues due to the Elytra...Wait does this apply to non-PvP games too? Because if a game doesnt have combat then 1.8 people shouldn’t have trouble changing version would they? So we can expect new arcade games with new features in the future?
A game like Slime Survival cannot be on 1.8 and it solely relies on the use of the glowing effect