surface=trail has no clear meaning, any plausible use can be expressed with existing surface tags (especially
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.