Is it possible to access referee reports for investigating scientific history?
All reputable journals today use web-based manuscript management systems that keep information about each manuscript, its various revisions, who was asked to review and what their reviews were, as well as editor decisions. So yes, this information exists in these systems at least as far back as the introduction of these systems (at least ten years ago). As the editor-in-chief of a journal, I can look at all of this data for every article in my journal.

But I suspect that it will be exceptionally difficult to get at this kind of information if you're not the editor-in-chief yourself. To do research on this means that you're doing research on human subjects, with all that implies: You'll have to have IRB [1] approval for your study protocol, you might have to anonymize the data (which I suspect would be exceptionally difficult in itself), and/or you might have to receive informed consent from the reviewers, authors, and editors (all of which are involved in the decision making process). 

I have a suspicion that few publishers are interested in going to this level of trouble. On the other hand, I have no doubt that such studies have been done, so I would expect that it is possible to get at the kind of data you're looking for -- it's just very difficult to do this kind of research.

[1] In the US, IRBs (Institutional Review Boards) are the bodies tasked with overseeing research that involves human subject research. All human subject research has to be approved by an IRB before the data so gained can be used for publications.