It's going to come down to 2 things: (1) Will your browser actually run Flash after December 2020, and (2) Will WGT continue to host the Flash-based version of the game on their servers?
The major browsers (eg. Chrome, Firefox, Edge) have already stated they will not support Flash after that date. So you'll be stuck either using an older / unsupported version of that browser (which carries security risks) or switching to a different browser that does.
But if WGT decommissions the Flash-based version of the game, all bets are off. In that case, you'll either have to switch to the mobile version or the new PC version (currently called WGT Early Access) to continue playing. WGT has not said yet whether they will do this or not.