I have just answered this question on SO, and it fired off my curiosity a little. So I've searched around but can't find an answer ('yes' is a difficult thing to search for!)
There are related questions about what yes
does, but I can't find any about its history, and I'm curious what is the first instance of? is there any documentation for it…
Best Answer
The earliest version in the Unix history repository is credited to Ken Thompson in V7, on January 10, 1979.
The 4BSD version only adds the CSRG's SCCS header, dated October 8, 1980.
In the repository, the V7 version doesn't have a manpage; that appears in 4BSD.