There are two issues that I could find related to these characters, one of them still open:  #18236 and #7643.  The latter contains a fairly complete discussion of the underlying issue, but on a quick read through it is not clear to me if the linebreak issue was actually completely addressed.  It would be good if someone unicode knowledgeable would read through that issue and see if the current state of affairs is in fact correct, and if so (as seems likely, given that there were unicode experts weighing in on that issue) we need to improve the splitlines docs at least (as was suggested in that issue but not done).  How tightly related that issue is to this one depends on how codecs and IO implement their linebreak algorithms

Perhaps we should retitle this issue "make Python's treatment of 'information separator' and other line break characters consistent".

Since backward compatibility is an issue, if there are changes to be made there may be changes that can only be made in 3.4.