All functions could have a started modifier as they should not be called before the start time
Description
Only Codeup::addGameETH() checks if the protocol has started in require(block.timestamp > startUNIX, NotStarted());, but the same checks could be applied to the remaining functions. Although this presents no vulnerability as these functions will not do anything before adding gameETH, it is a good practice to prevent unwanted behaviours.
Recommendation
Create a modifier having require(block.timestamp > startUNIX, NotStarted()); and place it in all functions.