Last time this was done, no exchange went down. So it was either not successful in it's goal or the exchanges outsmarted the customers.
Coinbase actually started to move its cold wallet holdings around the time proof of keys was announced. It happened well before the actual event, probably to let people know they are in control of all people's funds so that they don't have to withdraw any funds themselves.
https://blog.coinbase.com/notice-of-blockchain-movements-b09cb1ec46ddI find it hard to believe that it was coincidence. It could be, but the timing was very suspicious. I however don't think we'll see them do something similar this year. They must have noticed that there isn't much community support for such an event. You'll only see so called Bitcoin maximalists bark on social media, but I'm quite sure they aren't withdrawing funds themselves.