Author here - the main reason there's no examples is because I didn't have any interesting ones to report at the time! I was trying to develop new detection methods, but found only the (thousands) of undocumented software prefetches which were previously reported by Domas in his Sandsifter project, e.g. 0f 0d /2 and /3-7 on Intel CPUs (these are documented by AMD, but not Intel, and opcode behavior varies more often between the two than you'd expect). Many of the interesting undocumented x86 opcodes (e.g. icebp, salc, loadall) were either only present in older CPUs or are now at least partially documented. There are some much more interesting undocumented opcodes on other architectures (which have architectural effects, e.g. changing register values, halting the CPU), but that's still an ongoing project.

Edit: 0f 0d /2 is documented as prefetchwt1 but (allegedly) unsupported by the CPUs I tested it on, so the fact it executes at all is undocumented.