We have just had a great discussion about this in the Pro Access Pass membership meeting today... I thought I would follow up with some screen capture examples and share the knowledge for those who were not there.
Plug-in's that provide bands above 20 KHz are oversampled to prevent aliasing.
You hear a difference in the source not because you can hear (or perceive) boosts in frequencies far above the limits of human hearing but because the slope of these bands are extremely broad and commence well within the audible frequency range.
For example, take a look at this 40 KHz Air boost at maximum gain on Maag EQ 4, the band begins just below 2 KHz:
Here is Maag EQ 4 again with a 20 KHz Air boost at maximum gain:
Here is Manley Massive Passive with a 27 KHz boost at maximum gain:
Unfortunately, Mixbus has decided not to depict frequency and phase response contours for UAD plug-ins, so white noise will suffice.