Are you suggesting any player who has played since 2000 and has headaches or depression can sue the NRL or their former club for letting them play the game? That was when the first strong direct evidence started to come out about the role of concussions and ABI in contact sport such as NFL and RL. It would take an awfully good legal team to argue that is the case. When the evidence became strong enough to start to base clinical guidelines that led to practice guidelines for sports on is very debatable. The NRL started to put out guidelines around 2010/11 from what I can see and gradually firmed them up as the consolidated evidence started to show the potential harm and the risk of litigation following the NFL cover up became greater.
Again we have to be clear, you cannot sue them for getting concussions during a game. Only if they didn't tell you the risk, didn't do everything "reasonably" possible to minimise the risk, and you could have an argument around duty of care to provide follow up medical support at the time.
We have known repeated concussions to the brain cause brain damage since the early 1950;s, this is nothing new. What we have only recently found out is the more detailed events that can lead to long term damage, ie how many times, how often, repeated short time concussions etc. If this info is shared with any potential player and they decide to continue to play then it would be very hard to argue negligence by the NRL or a club, especially if every attempt has been made to minimise these risks.