Types of sampling and signal processing: some conceptual questions
The ZOH process is defined in Wiki and the sampling process is rigorously in enrico’s  link to @robertbristow-johnson ‘s excellent answer.
There is some need to filter the quantization harmonics of the DAC which adds noise beyond the Nyquist BW limited signal that was digitized. They define this as a rectangular ZOH process, not because of the S/H in the DAC because technically it’s not called this but, just a filter signal after a digital register that is periodically updated.
FWIW the wiki plot of ZOH time sampled signal looks like the result of a track &amp; hold ADC and not a DAC (implied) whose filtered response might change directions at the end of a sample after settling rather than the beginning due to group delay or is it step response (?).

I think it was Hartley’s Law that precursor Shannon’s that defined the the relationship between ideal sample rate and the ratio of baud rate and quantization levels.
Therefore in your question for understand, not that real Nyquist filters cause ISI and spectral suppression of high f signals that are far lower than main signal BW at -3dB.
To me it means the ideal sampling is not just the Nyquist rate or sampling with/without retention, but sampling at a rate to exceed Hartley Law for the GBW product of dynamic range times the BW and acceptable distortion, group delay, phase error or ISI of the restored signal.
My understanding is that oversampling makes use of this to enable the anti-aliasing filter to use a lower order filter as well for reconstruction using something like $$f_S &gt; \sqrt{(2^N)}* f_{3dB}$$
Perhaps I do not understand the  retention theory but normally this is the stored energy latency and group delay in a filter or the integration average of a bit rather than the Track/hold or multiplexed S/H characteristics of the sample.
Ref: https://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Zeroorderhold.signal.svg/220px-Zeroorderhold.signal.svg.png