My personal opinion is that what the IEEE discusses are by and large the mainstream tests. There are a wealth of other possible monitoring opportunities. But they have to draw the line somewhere.
I'll mention there is one very low-tech cheap partial discharge test you can attempt with the Draeger tube sampling equipment available from the Industrial Safety department of most industrial facilities: an ozone test. Any ozone above the detectable level (I think it's 100 ppb) in the motor discharge airstream is a bad sign. The likelihood of actually finding or confirming a bad machine using the standard ozone test would be 0 for 460vac machines, not much better 4kv machines, and a lot better for 13.2kv machines imo. I have tried it out during a shop ac hi-pot tests and it did detect ozone at about 10 times minimum detectable during that test. I also checked several of my 13.2kv motors during operation and didn't find anything above minimum detectable yet. I think the tremendous dilution created by the operating air-flow (compared to the stationary hi-potted machine) means that even if I had the same level of pd going on in my operating motor as was going on in that hi-potted machine, it would still be below minimum detectable due to the dilution. But thinking about it now, there is a way to improve the minimum sensitivity of the test simply by increasing the air volume drawn through a single tube. One of these days I'll get out and keep drawing air on one of my bad actor motors (indicated by traditional p.d monitoring) until I get an indication, then back-calculate the concentration. At any rate it's easy, quick and cheap to try. The ozone tubes costs about a buck each and are used up each time you do a test. The Draeger hand sample pump (if you don't already have it) costs maybe $100, but once you buy it you can use it over and over again.