My MC pickup is specced for 50 kHz (Van den Hul MC One Special) and although there are no +/- dB limits mentioned I'd expect it to reach 20 kHz with ease. Yes, you cannot do much to improve FR but I think you don't need to.
Any of the micro ridge styli (regardless of cantilever type) will be able to trace out to beyond 40kHz - they were originally designed for it as part of the development for Vinyl based Quad records.... (Shibata was the stylus designed for it at the time)
But the effective tip mass will drive the cantilever resonance - as an example - the Shure V15V's used a Beryllium Tube cantilever - the end result was a resonance at 32Khz (with some variation based on manufacturing variability)
A Solid boron cantilever is highly unlikely to ever achieve as low a tip mass - and therefore the resonance will come in at a lower frequency - typically at around 19kHz or 20kHz - heavily dependent on the design of the mounting system and its impact on overall mass.
My 10 year old Boron cantilevered Jico SAS needles, measure the resonance at 14kHz and 16kHz respectively (much lower than I expected)
And yes - they can track and provide good quality signal well into the ultrasonic (I have measured out to 48Khz) - but that doesn't tell you about the rise and falls in that response.
It has to be measured - and with MC - typically you will see a rise in the top end - as it rises up to the resonant peak - a bell curve... even on the Shure V15V's you can see the start of the rise to the 32kHz peak at around 19kHz.
Technics did manage to manufacture some ultra light Boron pipe (hollow) cantilevers with resonance at around 50kHz - as fitted to the EPC205 and EPC100 cartridges.... unfortunately long gone, and no one is manufacturing this type of cantilever (to the best of my knowledge - would love to be corrected!!) - their response out to 20kHz as a result was ruler flat... the beginning of the resonance rise was already in the ultrasonic range.
So yes - reaching 20kHz with ease... I'm sure it can and does - reaching it with a flat F/R - I seriously doubt.
P.S. I have a near cousin to that cartridge, the Empire MC1 - exact same body, but with a much more ordinary cantilever/needle - Empire at the time partnered with VdH - not sure which member of the partnership manufactured the cartridges, as they look identical with the exception of colour and labelling.