Sure, online polls have obvious flaws (non-response bias, how many people actually saw the poll, who was motivated to actually respond to the poll, did voters actually understand the protest would last more than 1 day, should logged out lurkers have any representation, etc)
But beyond that, shutting down a subreddit for multiple days is such a drastic action, it should require more than just a simple majority in a quick online poll.
For instance, to amend the Constitution, you typically need a supermajority (2/3 or 3/4 of different parts of the government). To convict someone guilty in a trial, you typically need evidence beyond reasonable doubt and a unanimous jury verdict.
The burden of proof that users want to shut down the subreddit should be overwhelming.