Can EGR go bad?

Likely head gasket has failed, is leaking coolant into cylinders, and the “death rattle” is due to engine’s struggles to compress the coolant.

that was the first EGR cleaning?

 

I agree that the likely problem is that the head gasket is starting to fail.

In the early stage of the leaking head gasket in the Gen 3 engine, the "death rattle" is due to misfires caused by the leakage of coolant into the cylinders. The coolant cause the spark plug to create a weak or no spark from the plug until the coolant is forced out through the exhaust valve of the cylinder. Then, the spark plug will work properly, and the misfire will stop occurring, and the engine will run smoothly again.

In the advanced stage of a leaking head gasket, you could get enough coolant into the cylinder for the engine to struggle to compress the coolant in the cylinder. Once that occurs, you will bend or break the cylinder rod. If that occurs, it is time for an engine replacement.

 

There's probably a continuum involved, where enough coolant can collect to effectively raise the compression ratio and make the compression stroke harder to complete, but before it gets so hard as to break something.

But it's worth remembering that none of that is necessary to make the awful rattle. It is a big noise, but does not need a big explanation, and even simple misfires in a Prius cause that noise. This classic story concerns a Prius that was let go for scrap price when the awful rattle came from misfiring caused by a single spark plug, and there was a quite recent thread here where someone seriously considered letting the car go for scrap price and the cause of misfiring was an injector.

The engine is connected by fixed gearing to two massive motors (and one of the massive motors is further connected by fixed gearing to the whole mass of the car). Misfiring makes the rotational speed of the engine uneven, and the car isn't able to control the motor rotation precisely enough to match those variations, and there is always some lash in the gearing that can clunk back and forth. There's a torque damper between engine and transmission to make the clunks a little less bad, but that takes its toll on the damper too.

That's not to say your trouble is a spark plug or injector. If there was white smoke, head gasket sounds like a good call. The point is just that the misfiring caused by coolant's presence in the cylinder is already quite enough to explain the awful noise.