There are a number of threads on the Mystara message board currently discussing Shadowdeep, it is a work in its inception at the moment - the following are some articles which may not be fully fleshed out yet. The threads thus far include:
Rockhome
Shadowdeep prestige classes
NPCs of the Shadowdeep
Races of the Shadowdeep
Regions of the Shadowdeep
Magic of Annwn
Shadowdeep
Underworld extension by Trenton McKinnon, from the D&D BECMI Facebook group posted 15 June 2024.
Darmouk by Giampaolo Agosta and John Calvin, from Threshold Magazine issue 16 dated 28 July 2017.
Through the Shadowdeep, from Karameikos to the Hollow World by Francesco Defferrari, from Threshold Magazine issue 14 dated 22 January 2017.
Deep Hollow, a creature crucible for the Shadowdeep by Giampaolo Agosta, from Threshold Magazine issue 14 dated 22 January 2017.
The Unknown World Trail map - part 2 (Broken Lands and Shadowlands) by Francesco Defferrari, from Threshold Magazine issue 14 dated 22 January 2017.
The Cavern of Thirith by Rodger Burns, from The Piazza posted 7 June 2009.
Wirrtenheid by Lost Woodrake, from the Mystara Message Board posted 9 June 2004.
Magic of Annwn by Lost Woodrake, from the Mystara Message Board posted 9 June 2004.
NPCs of Annwn by Lost Woodrake, from the Mystara Message Board posted 9 June 2004.
Regions of the Shadowdeep by Andrew Theisen, from the Mystara Message Board posted 8 June 2004.