By the time that the “Key:informal” page was created in April 2011, there were already some 889,000 highway=path ways stretching 258,823½ miles (416 536 km), including 49,900 ways stretching 22,027 miles (35 449 km) in the United States alone. It was not until mid-2019 that informal=yes usage passed 5,000 ways.
In any event, passive voice may be causing some confusion here. I suspect @n76 isn’t referring to @dieterdreist’s initial documentation. Rather, lots of trails had already been mapped and presented to users by 2016 or so, when others identified a problem with social trails in the U.S. and various community members from around the world insisted on a mix of informal=yes and access=no as the solution. Even more trails had been mapped and presented to users by 2021, when U.S. trail managers’ frustration with OSM-based applications boiled over and we began to prioritize the problem.