CD sampling is 44.1kHz. 22.05kHz is used in plenty of HAs. I haven't seen anything lower (but wouldn't be surprised to learn otherwise)

22kHz sampling isn't a problem in practice as:

- HAs focus on speech comprehension, not music. Speech doesn't high much content above 5kHz, so even 22kHz is double the Nyquist rate.

- Music is perfectly enjoyable at 22kHz (but obviously better at 44kHz)

- People getting hearing aids have hearing loss, and the majority of hearing loss is in the high frequencies