A commemorative plaque on the ground, if it is a plaque and not other type of memorial, should be tagged as historic=memorial + memorial=plaque.
A paving stone is not a plaque, so historic=memorial + memorial=paving_stone (or something equivalent for a good English speaker) is OK for me.
Some objects used as support for memorials are very rare. It is not necessary to modify the wiki documentation to include all possible values (it wouldn’t be useful). Taginfo is a better tool to know other values in use.
A traffic light memorial to Chiquito de la Calzada in Malaga, Spain. Source: Wikimedia Commons.