The answer is simple, and entirely unclear, I'm afraid.
There is no generally accepted delineation of when you shouldn't use the term "step-parent" for the new spouse or partner of a biological parent. I think there's general agreement that you should when you live in their home as a child, and perhaps even for the spouse of a parent with whom you do not live, when you live elsewhere but are still a child. Use by adult children, however, is largely left up to individual preference.
There are likely regional trends. Certainly, sometimes choosing to use the term "stepfather" or "stepmother" connotes closeness, and other times it indicates a resentment for that position. It's difficult to navigate, but I don't think most people would ever think someone was absolutely wrong to call someone their stepfather or stepmother unless their relationship was not, in part, defined by someone being the parent of one and the spouse or partner of the other.
The comments raise the question of foster parents as well as step parents. Those should not be confused, but they are describing very, very different things. Foster parents are those you live with as parents while having no close legal or blood relationship. So, for someone to be a foster parent, you must live with them. However, they will not be a foster parent if they are also a parent or step-parent.