They usually do. (The considerate and/or non-confrontational ones. There are always idiots, and people have the tendency to remember negative outliers and project their behavior on the group as a whole, which is unfortunate.) However, slowing down isn't the whole story. Riding a non-motorized bicycle is much easier if the rider can keep moving, however slowly, so it would be considerate in turn for the pedestrian to step aside and let the cyclist pass, if possible. A distracted pedestrian can be warned by a bell.
Separately, delivery riders as a category have an incentive to ride as quickly as possible, which is a recipe for conflict. Removing that incentive means removing or completely reimagining the service. I don't think that anybody has a solution or mitigation at present.