I've received emails from a couple of casinos that had the same message: the dates being extended. It appears that these casinos will start a new,six month clock starting with the date they reopen as long as your tickets weren't previously expired before the shutdown.
In other words if your casino shut down on March 15 and your ticket expired on March 16 you have six months after they reopen.
But if your ticket expired on March 14 you don't get the extension.
Station Casinos has this on their website:
We are extending the expiration date for the slot machine cash out vouchers for 6 months from their current expiration dates, so long as the voucher expiration date occurs on or before March 31, 2020.
I think that by now every casino has info like this on their websites.