Computing history time!
While Open Source was a thing before then, OpenBSD was the first group to have readonly, anonymous, and immediate availability of the source code (as changes are being made) for everyone, with anoncvs in 1995.
Before anoncvs was available, many groups would release a tarball of the source code, or the whole repo, next to the binaries.
song - https://www.openbsd.org/lyrics.html#61
paper - https://www.openbsd.org/papers/anoncvs-paper.pdf
slides - https://www.openbsd.org/papers/anoncvs-slides.pdf