I'm running a multi-monitor setup using i3 and configured polybar to just display the workspaces of the current monitor (pin-workspaces = true). When a window gets the urgent hint (e.g. a messenger receiving a message) the workspace is highlighted, but only on the bar of the monitor where it happened.

It might be beneficial to add a toggle to allow to show urgent workspaces on the current bar even when the workspace itself is not on the current monitor (this just makes sense, if pin-workspaces = true is set).

I've implemented this already here. If accepted I'd open a PR.