But for the most part it contributed massively to the happiness of the developers. And the outcomes, in my opinion, were invaluable. It's not always visible from the outside, but Atlassian now has swathes of valuable internal tooling, built with love by developers who were invested in solving their own productivity problems.
The quintessential "20% project" is GMail but I think that misses the incrementalism that 20% projects really provide. Developers will absolutely take advantage of the time to improve their personal ergonomics, and everyone around them benefits from that.
But this is obviously very difficult to measure.