So my guess is that (a) the Google Sets feature is discontinued, much to the dismay of the implementing team, (b) they (you?) see a lifeboat in putting this into Google Docs. It's way better there than deep-sixed entirely, but Google Sets was pretty useful to me as a standalone tool - cleaner UI for the capability.
They shut down the consumer frontend to Google Sets but there are clearly internal instances still running to power this feature (it may also be used in coming up with related terms in another ad tech product, but I'm not sure).
I was new to the Google Spreadsheets team back in 2007 and autofill was a feature that had not been implemented yet. We thought it would be a cool thing to make it a bit better by adding Sets support, so I worked with Simon Tong—the author of Google Sets—to get it playing nice with autofill. To be clear, I did not write Sets. Simon did.
This feature has been live since 2007. It was not created to be a lifeboat for Sets and the frontend getting shut down is the result of (my guess) low user traffic.
