pdftex character protrusion without latex: what happened to protcode.tex?

If one uses pdftex, not pdflatex, and wants character protrusion, it used to be that one would input protcode.tex. That was present in texlive2015, but disappeared by texlive2019. The pdftex manual (Rev. 790) does not mention protcode.tex, although the microtype package documentation does for historical purposes. What explains its removal? Is there a substitute, or is one now expected to implement one’s own decisions instead? (I do have a saved copy of protcode.tex.)