I just started up the game for the first time since the update, and right-clicking while dual-wielding now raises the off-hand weapon in a block rather than attacking with it. Has anyone else noticed this and/or figured out how to attack with an off-hand weapon?

I don't have this issue and I have literally no mods installed, so it sounds like an incompatability of some kind.

It isn't a "new feature" in the patch then?

Nope.