Reason for MPD is that it is a backend for myMPD, O!MPD etc, so commonly not installed on its own. But still makes sense to add it, at least to mention how we configure it by default (“default” ALSA PCM) and that it can be further configured with dietpi-justboom (dietpi-config submenu), cross-linking CAVA console visualizer etc.
We could also add platform compatibility/availability to the docs, but I fear the additional maintenance efforts this causes. First we should implement some central information system, from where website, docs and probably even DietPi scripts grab such meta info. If someone has time and mood to work on such 
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