I think this all goes back to work by Michal Fagan at IBM.
I have vague recollection of seeing an article way back in the late eighties by him in the IBM System Journal
This article in CIO seem to imply that the idea of the relative cost of fixing software defects escalating as one progressed through the SDLC was originally documented by Fagan.
https://www.cio.com/article/2399877/rethinking-software-development--testing-and-inspection.html