Hi Martjin,
Yes, this is the first thing I checked. In fact I checked it in GA > Acquisition > Search Console which enables you to filter the data quite well by regex. What I found is that it's pretty much across the board with the homepage as well as individual product pages taking a hit of around 50% reduction. It's definitely NOT traffic on a few higher volume keywords as the major drop that's hurt quite a lot is traffic to individual product pages. That indicates to me that it seems to be some kind of a site-wide penalty. I did also notice that traffic to product print pages (where the product is in a print-friendly format) effectively went from say 300 to like 30 (and probably not much can be done there), but it's the overall traffic from SERPs that's more concerning, particularly to long-tail product pages. So I'm pretty confident it's some sort of a penalty, but no idea why it's would be applied besides what I've already flagged in my original post 
