Arguably git still does this: quite a lot of the implementation is exposed to the user. But, as someone who learned RCS: that paper was exceptionally clear and

Arguably git still does this: quite a lot of the implementation is exposed to the user. But, as someone who learned RCS: that paper was exceptionally clear and the whole thing was extremely comprehensible, right down to the ,v format being human readable. In retrospect VCS had a long way to go, but going from nothing to RCS was huge.