The Onslaught has not changed in the past couple of versions, as far as I can tell. Attacking it on the forward ~150 degree arc is and always was a bad idea for anything that isn't a Paragon, another Onslaught, or a battlecruiser piloted by a character with a significant level advantage. Attacking it on the flanks is a bit more survivable for the more durable cruisers, but hitting the rear ~150 degrees (of which roughly 100 degrees are covered by only four medium ballistic mounts and nothing else) has always been and remains the best way to tackle the Onslaught if you don't have a battleship of your own or a significant fleet. The Onslaught also makes up for its armor to a degree with its shields, which are more of a liability than an asset in many circumstances.
Also, as far as I can tell, the story isn't that weapons-grade energy weapons are new. It's that weapons-grade modular weapons are new enough that they're uncommon on Early Epoch (low-tech) hulls, and that the TPCs mounted on the Onslaught are there in part because whatever the modular energy weapons available at the time the design was created were, they weren't considered to be good enough to mount, and so the Onslaught was given permanent TPCs rather than modular large energy slots.