When I get my money out of the account because of deflation, why would I choose cash over, for example, gold? In financial unrest, holding cash gives zero return at best, and more likely a large negative return due to inflation.
Lately, the Austrian central bank was rumoured to play with the idea of negative interest rates so the Austrian pension funds quickly brought up the idea of just storing their paper in secure vaults. As long as the costs of storage / protection are lower can the losses due to the negative interest rates, it's a net gain for the funds.
The central bank isn't directly allowed to prevent anyone from just getting their money in cash and while they could delay it by "process", only the government itself could issue a blockage on cash deposition.