That wasn't only a 20% project but a secret one! For several months I told my manager in daily standups that today was my 20% day, and that it was related to what the team did, but not what the project actually was. Everyone was cool with it.
It's possible that this was achievable only because I worked in an operational role. We had firefights that sometimes disrupted my ability to take the 20%, but you could partially bank the time and there weren't top-down imposed project deadlines.
I also used and experienced many other people's 20% projects whilst I was there. Many were small internal tools and systems that made working life easier but weren't the right size/shape to be staffed up as a full project with management attention, but some were products. At one point someone did a project that let you bid against yourself in the ad auction, to experiment with how to pay to remove ads. IIRC that was a 20% project. Google's early reputation for ML skill was established on the outside by Google Sets, a (for the time) astonishing demo that revealed Google was no mere fancy keyword matcher but had semantic understanding of words too. Pretty sure that was also a 20% project.
I felt pretty strongly at the time that the 20% policy was one of Google's secret weapons. The disdain and disinterest with which it's so often greeted outside of Google's walls is puzzling and sad, as is the quasi-myth that it didn't exist at all. I can believe that it existed to varying degrees depending on where and when you were, but, it's definitely not a myth.
