The fields of the index structure, such as "label", "containingModule",
"containingPackage", "containingClass", "holder", and "description" are
abbreviated in the index files to a single letter, "l", "m", "p", "c", "h", and
"d" respectively. The "url" field is not. Sometimes it is referred to as "u",
other times as "url".

From my archival research it seems that this peculiarity has been there from day 1.
While "url" was used for indexing packages, types, members, and then modules,
"u" was used for everything else, which back then comprised the "search tags"
category. I couldn't find any evidence of that being done for some purpose.
I do believe it's just a typo, thus I propose to fix it.