From 1990’s people like @mralancooper & Grady Booch - https://en.m.wikipedia.org/wiki/Grady_Booch have been designing & building tools, writing books to address the human developer eXperience (DX). AFAIA the term did not become popular until Software as a Service (SaaS) with a focus on User eXperience became a business interest. Then certain businesses realized their SaaS products & APIs needed developer adoption for success and DX became a base SaaS business consideration - https://thenewkingmakers.com