Whether you construct the substructure as drywall, with Ytong or a wooden frame + OSB, etc., is almost irrelevant. If there are concerns about warping of the staircase system, in my opinion, one should simply not connect this new component directly to the staircase. You could deliberately leave a gap of, for example, 1–2 cm between the built/masoned base cabinet and the wooden staircase, or in the case of a wooden construction, simply place a strip over the gap, which can then be repositioned or adjusted if necessary, in case some warping actually occurs.