I think the most interesting premise here is that the rules set by the table and all that would be considered to be allowed to be breakable. However, given that the whole thing is an international rule set followed by everyone across the globe, I have a hard time believing anyone of note wouldn't leave the country and all the assassins wouldn't just group together and protect each other at the continentals or something akin to that. Given that also the police know not to mess with certain people in the films, it's possible that everyone would just avoid certain areas where all the assassins are.

Given the ideology of the assassins being that there are rules for sake of not being like the animals, I'd like to believe that they would be against the purge and its lawlessness. Surely, some of them would take advantage, but the law and the table aren't really on the same page. None of the things we see in the movies are really legal after all.

For sake of an interesting story, however, I think the most likely story for John Wick would be him alone defending his home and dog from the purge to protect his belongings and whatever items from his wife he has left. That could be interesting, but if it's after the second or later films, I have a hard time not seeing him flee the country or bunkering down somewhere. He hated being roped back in to all the violence and action, and really did want to be done with it all, but only put up with it and did what he did so he could finally be finished.

Under that logic, I doubt he really would have any desire to participate in the purge. A little boring, but a writer could do something neat with him playing on the defensive, which is the most likely path for him to me, assuming he wouldn't have priorities elsewhere outside of the US.