BaSiX – A Basic interpreter written in TeX (1990) [pdf] (tug.org)

I suspect this is just the code equivalent of performance art (with the emphasis on art). People do a lot of funny things just because they can, and, since source is embedded in the paper, I’m guessing this wasn’t just an April Fools prank.

Also in the tradition of ambitious TeX is this 2008 winner of the Judge's Prize for the ICFP Programming Contest: http://sdh33b.blogspot.com/2008/07/icfp-contest-2008.html (contest link: http://web.cecs.pdx.edu/~sheard/2008IcfpContest/).

It looks like there's a copy in CTAN: https://ctan.org/pkg/basix