I can't find it on mobile right now. It was a preliminary result, because the attached caps haven't been required that long. Also most countries have implemented extra pfand systems for small plastic bottles, which also helps. So it's tricky to say which regulation helped most.

But there are plenty of sources of how many bottle caps there are in the streets and oceans. And how harmful they are to animals who think the small bright things are food.

It also makes sense, instead of the bottle and the cap becoming seperate pieces of trash, it's now a single piece. So it reduces the number of pieces if not the volume of trash.

Also for people from the US where bottles are shredded and caps typically not recycled. In Europe the caps also don't get recycled, but instead removed. The bottle is then checked for leaks and defects and if it passes it's cleaned and then reused. Actually recycling plastic is hard, so this way a bottle can be used at least two or three times.

You don't get your pfand in many cases if the cap isn't on, as well as the label many times. This isn't so much needed for the recycling but important for the whole process. For example groceries are required to take in bottles, but are allowed to limit this to bottles they sell. So the barcode om the label is checked for this purpose. The label is removed in the recycling process. The cap is required to motivate people to return those and not have them turn into litter. It's also a hygiene thing for the people handling the bottles, often there is liquid still in the bottle and without the cap it comes out during handling.

I will find the data later when I'm on desktop.