We are using old style objects in a few places, but also using the slots feature which is only available for new style objects.
See for instance this alert the small help icon to the far right links to a more detailed explanation.
These were found using LGTM.com, the producers of which I now happen to work for. I would be happy, if it could turn out to be useful for blender :-)
I just assigned a single reviewer, feel free to push this around...