initial documentations/articles that list the primitives
    needed and what they have to achieve?

I am not sure if it is complete, but there was a list provided in the
LaTeX news of the time:
As for documentation, the pdftex manual, at least, describes them all in
its usual terse way. Not sure about doc for the other engines. If the
pdftex doc needs improving, let me know.

LaTeXers - personally, I think it would be nice if the "primitive
requirements" became a permanent part of the LaTeX
documentation. (Apologies if they already there.) Maybe?
    rather than any technical restriction: pdfTeX is GPL. 

FWIW, I suspect Thanh would not have a problem with allowing use of
those primitive implementations under CC0 or any other license terms.

Thierry, let me know if I should ask him ... --best, karl.