This DLC was avalible for free during the (almost apocalyptic) release of the PC version, both in GOG and Steam. However... it's completely absent now. I was expecting the DLC to be paid after the grace period, but that doesn't seem to be the case. It's not included in the "Ultimate" Resource Bundle, not in any free DLC or the strangely standalone Luxia's Eternian Scholar outfit.
So... what's the deal? Can I purchase the DLC in any way? Can someone reupload it, since it is (was) a free product?

Considering gogdb the DLC is exist but nothing else known lol.

gogdb

Yeah, I know, I tried downloading from there but despite figuring as unlisted it's actually private, so it always throw a 403 Forbidden.
I don't know how questionably legal is, but a reupload from someone who has access to the DLC seems to be the only way to get it nowdays... Again, it's not a paid DLC, it never was, but I guess it was included for a limited time for a reason (to bait us into buying that disaster of a port, that's the reason).

Gogdb is basically the same as the standard GOG website and work the same way. Just like here, when there is a free thing you still have to actually buy it even if you pay nothing and can be downloaded after that. Yes, this doesn't help.

Anyway this DLC was a preorder bonus. I really know nothing about the PC version but the PS4 store offers it free even now. I guess you can try looking for the PC version of the DLC since it is free everywhere as you said so shouldn't be illegal sharing it. Honestly the quality of the DLC not really worth the effort though. I'm pleased with the performance of the console version but the silver armor (the reason for this DLC) isn't as cool looking as it could.

Yeah, I tried to find it to download, but no luck... I actually have the installer of the DLC for a much older version of the game, but obviously it doesn't work with the current version. I think I might try my luck DMing some people here on GOG that have the game since launch.

Oh then you already solved the problem for you. You can use a program called "innoextract" to extract the files from the installer. The files weren't updated since the release and there is no new version for the HD texture pack either. As far as I know the DLC not updates the game's .exe file. Technically only the installer got updated. You just have to copy the extracted files corresponding the other DLC's structure. :)

Thank you, I'll try that! I unpacked it and so far I only managed to hang the game at the "Checking downloadable content" screen, but I'll figure something out. Thank you very much!

No problem. I guess it just need to be in the right folder structure to work properly.

No luck... I edited most config files that listed the DLC and included its own files (and upgraded them to have the same structure as the other DLCs), and it still hangs when checking the DLC. I also rechecked gogdb and the patch recieved an update that altered its size around january, so we can assume that it changed in some way other than the version number. I'm out of ideas, honestly. But again, thank you for your help.

Then something really got changed recently. Sorry for not being able to help here. With a bit of luck maybe someone can send the up-to-date DLC, Or you can try asking the GOG support directly for some advices. Problems like this is my reason for playing on consoles rather than on PC.