Quoted from the OSM US Trail Stewardship Initiative Wiki page. For tagging: "A social trail, and access to this social trail is not prohibited outright, but is officially discouraged by the land owner/manager.
(as evidenced by posted signs or markings, or by text on e.g. the official website or visitor maps)

highway=path
informal=yes
access=discouraged"

Link to page: https://wiki.openstreetmap.org/wiki/United_States/Trails_Stewardship_Initiative