I also didn't watch this video here but saw videos earlier where livestock was fed with expired food that supermarkets weren't able to sell in time. All kinds of products like vegetables, bread, cake, yogurt etc. were delivered to the farmers directly or to intermediate companies. In many cases the food was still in its packaging, e.g. a plastic bag around a loaf of bread. But to keep costs low everything just went into a huge shredder and was then fed to the animals. Including lots of micro (and not so micro) plastics.

So the cost cutting is not about "explicitly feeding plastics as a cheap filler" but rather "accepting to have plastics in the food in favor of lower sorting costs".

In the documentary I watched this was described a common practice all over Europe.

They're not specifically feeding them plastic only, he's showing how any waste food (old potatos, old bread, chips, etc) that comes to be processed does not have the plastic bags removed before going into a grinder to become feed.

He actually shows how it's explicitly allowed in his state regulations, likely approved due to the efforts of an agribusiness lobbyist. I suspect that any negative effects to the pigs is not enough to effect the end product/bottom line, or doesn't manifest within the timeframe of a viable animal for slaughter.