In the docs https://getbootstrap.com/docs/4.0/utilities/position/ it's mentioned that Quick positioning classes are available, though they are not responsive. Why is like that. it's a handy utility class and it's more convenient if there are responsive utilities too.
There are many utilities that do not include responsive variations. There's not a high demand for all of these, so we kept them out until we heard more from the community on them as they add a significant amount of weight to the compiled CSS. For now, going to leave as-is though.
I'd very much appreciate the ability to use the position utility classes responsively.
As mentioned here the class names could be kept consistent with the p-[breakpoint]-[value] pattern used for the Display utility.