Module: AtlasLootClassic PVP
Category: Warsong Gulch
Loottable: Exalted
Items: Outrider's Plate Legguards

It's available in phase 6, not yet!

As Blizzard stated here:
https://us.forums.blizzard.com/en/wow/t/wow-classic-content-unlock-february-12/406222
The Argent Dawn, Thorium Brotherhood, Timbermaw Hold, Silverwing Sentinels, and Warsong Outriders faction vendors now offer reputation reward items that were added in patches 1.6 and 1.7 of original WoW.

The leggings were added in patch 1.11 stated here:
https://web.archive.org/web/20060719020411/http://www.worldofwarcraft.com:80/patchnotes/patch1p11.html
Warsong Gulch - Epic leggings have been added to the Warsong Outrider and Silverwing Sentinel supply officers for all armor types. These items require Exalted status with their respective reputations.
And patch 1.11 is naxx-patch, that means phase 6.