I've spent more hours than I'd care to admit digging through the internet for good Dark Age of Technology fics, and honestly, it's a bit of a desert compared to the mainline 40k stuff. The big names like Archive of Our Own (AO3) do have a dedicated Warhammer 40k fandom section, and you'll find some stories tagged with 'Dark Age of Technology' or 'DAoT' there, but the volume is tiny. It feels like people are more comfortable writing about the grimdark 'now' of the setting rather than the mysterious, almost mythic past. I've had better luck on SpaceBattles and Sufficient Velocity forums, weirdly enough. Those places have more of a speculative fiction and worldbuilding culture, so you get these long, detailed threads where someone will post a snippet about a DAoT archaeotech discovery or a 'what if' scenario involving the Men of Iron. They're not always traditional narratives, but they scratch that itch for exploring the lore's golden age.
Reddit's r/40kLore sometimes has inspired-by posts or short stories in the comments, but you have to wade through a lot of discussion to find actual fic. The real challenge is that the Dark Age is so deliberately vague in the canon; it gives writers a lot of freedom, but also means there's no solid foundation of characters or events to build a fandom around. Most stories tend to be one-shots or crossovers where a DAoT ship or AI pops up in the 41st millennium. If you're searching, I'd recommend looking for tags like 'Pre-Imperium', 'STCs', or 'Men of Iron' on AO3, and just accepting that you're in a very niche corner of an already niche fandom. It's a treasure hunt, but finding a good, multi-chapter story focused solely on that era feels like uncovering a lost relic yourself.