Meat and Dairy utensils

Question

You mentioned that technically since glazed china doesn't absorb one could use a single set for meat and dairy, but that the long-standing minhag is to use separate dishes. Does this only apply to glazed china or to everything? E. g., if I served something meat on a plastic plate, am I now prevented from using this plate with dairy? If not, why does the minhag only apply to glazed china?