Just installed a new dishwasher and hooked the drain hose up to the garbage disposal (knocked out the plug and clamped it to the inlet).

The previous owner’s setup had the dishwasher draining into a Y-fitting under the sink. That Y is still in the drain line, but now both branches are just open since it’s no longer being used.

Is there any valid reason for that Y to be left open, or should I cap it? It seems like it should be sealed to prevent sewer gas (or a potential overflow, if that’s possible?) but I want to make sure I’m not missing something before I mess with it.