Welcome to Librarium Online!
Has anybody/GW done one since the Citadel Journal one in 3rd ed? I want to make an updated/improved version of the one that was on the GW website.
Supposidly with the release of the new harlequin units and the buzz the people over at GW new they would drum up, there is some talk of making a Harlequin codex. I have a feeling it will be a spinnoff 'dex like the "kroot mercenaries" and others in the same vein (PDF only). I wouldn't look for it any time soon...maybe around the summer.