I think to say that prehistoric imagery as a whole is abstract or abstracted is being somewhat ignorant – there are definitely some abstract aspects to prehistoric imagery / art, but not all of it falls under that category. I think the most abstract aspects of prehistoric imagery are the things we still cannot explain today or even begin to speculate about (ex: shapes in Newgrange). However, some of it is rather straightforward and we can narrow down the potential goals the original ‘artists’ had when creating the piece, such as the pottery found under the ‘Neolithic Pottery’ link, which is speculated to have been used as a mortar in ceremonial practices.
If we are to strictly observe the cave art itself as the ‘prehistoric imagery,’ then I believe it to only be slightly ‘abstracted’ and not entirely ‘abstract’ (haha). Once again I think the cave paintings would be mostly used for historic purposes (intentional or not) and storytelling, with the occasional ceremonial reasons. I am not sure how much symbolism or any of those abstract concepts could be thrown into them, because, similar to a story, the audience is what gives the piece meaning.