To be honest, I don't think there'll be any games yet that won't work on 3.15 or 3.20 (which I believe is safe to upgrade to).
The PUP on disc is provided by the SDK the developer was using to build the disc image, so most developers will be using 3.20 or older. From what I've been told, SDK 3.30 was released about a week ago, although it's far too quick for any games using this SDK to make it through Sony QA.
Any games written with the 3.20 SDK should be safe to upgrade using the PUP on disc because that was released mid-February, well before the 3.21 rush job to remove OtherOS.
Of course, that's assuming that Sony don't change the PUP on disc for developers...