Back in the day, EHT probes were quite expensive, & the users paid due care & attention to the use and storage of them.
For this reason I would trust an old HV probe. (After, of course, checking it thoroughly for physical damage.)
Most of the old style ones are made from hard polystyrene, which, if looked after well, maintains its voltage rating over many years.
I do remember seeing one probe machined out of solid Perspex, but that was an outlier.
Most HV probes were used to look at TV set EHT, which although it has lots of voltage, cannot source enough current to do much harm.
Transmitter HT supplies are another kettle of fish, but most full size TV or Radio Broadcast Tx have their own built-in metering for such supplies, so even at such a site, the HV probes didn't get that much use.
3D printed devices are not as homogenous as cast or extruded plastics, but whether that is a problem, or not , I don't know.