surface=trail
has no clear meaning, any plausible use can be expressed with existing surface tags (especially surface=unpaved
and surface=ground
and surface=earth
)
It is almost entirely used by two people which are inactive, not responding to changeset comments and heavily used it in limited areas.
Out of 5880 currently present uses - 5417 were added by one account and 209 by another. Next account is at 31 uses of this tag.
I propose to document it as deprecated on OSM Wiki.
See osm tag history and Tag:surface=trail - OpenStreetMap Wiki for further documentation