I was just about to post a feature request for this…
Currently, zips I (naively) created on Windows with file.getRelativePathFrom(otherFile) open up on Mac with backslashes in the name.
As @yfede says, the spec does not allow backslashes, so I would call changing the current behaviour a correction. But at worst it’s a very nice feature for JUCE to take care of us if we don’t get around to reading the zip file specification. It’s on my list, honest.