Article 690 question. Is MC cable required for 48 VDC running from 2nd floor to 7th floor Observation Deck?

As long as the breaker is simple with no electronics in it like a GFCI, AFCI, or adjustable trip, then voltage doesn't matter and it rating is a max voltage rating. You can use them at lower voltage.
Yes at lower AC voltages but they need additional markings to be used at any DC voltage.
 
DC breakers usually have a larger open gap between the contacts (or are rated at much lower DC voltage than AC voltage) since you don't have any zero crossing on DC to help extinguish the arc
 
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