Except that it almost certainly happens on a small level with individual gateways. Asmodean said he'd never seen a weave like Rand used to block a gateway, and only very few individuals could perform it. Considering Asmodean fought in a number of large scale battles in the War of Power, I find it unlikely that he wouldn't have come across a ward that encompassed a whole area against Travelling. Most likely he missed it because it was a rare Talent used only by a few Aes Sedai, and he never witnessed it.

You can Travel from anywhere where there is the Pattern. No warding could stop that. RJ said that gateway blocking occurred on many levels, and we've seen Rand block a gateway and how it's done. In a battle someone would try to slice his weave, and he would try to slice their's. It would be a whole battle in its self. If Demandred isolated the battlefield from Travelling he'd put himself at a huge disadvantage in case he needed to escape.