I guess you're right, Max, it's a purely dynamic phenomenon, thanks for the correction!
I forgot that deep depletion can only be achieved by a dynamic voltage slew rate which overtakes the charge generation time, when no charge injection from an adjacent junction is possible.
If such a dynamic state is generated by a MOS capacitance (I think it is also possible with a reverse-operated (PIN) diode), there's probably a practical limitation on the substrate's doping concentration: the space charge region's thickness must be large enough to take over the lion's share of the voltage in order to save the MOS oxide from breakthrough, and so with the unfavorable dielectric constant ratio of 3 between Si and SiO2. I.e. with a MOSCAP, deep depletion can only be achieved on a lowly doped substrate; at a high doping concentration, the MOS oxide is likely to (destructively) break through before deep depletion can be achieved, I guess.
Pls. tell me, if you think I'm wrong again!