Don’t bother trying to learn the new shibboleths. By the time the majority has accepted them, they’ll be outdated and the progressives will have moved onto another set. Before ‘developed’ and ‘developing’ we had ‘first world’ and ‘third world’.

First World were countries aligned with the United States, Second World were countries aligned with the Soviet Union, and Third World were countries who were "neutral" or not aligned with either major superpower. Many Third World countries were newly independent, and were economically struggling but also wanted to assert sovereignty. Many were given aid packages to attempt to court them to one side or the other. The terms are outdated because of reality, not because of "progressives".

We didn't, though! At least, not for that purpose. When India's PM Nehru said to the UN, "We are the third world," it was a profound statement that had absolutely nothing to do with economics. "Developing" and "developed" were introduced to the public conscious as a desperate attempt to stop the ignorant masses (including, at one point, me) from ruining a useful descriptor.