This is a lightweight alternative for Goldberg for the single purpose of unlocking DLCs. Just rename the game’s steam_api.dll to steam_api.orig.dll, download steam_api.dll from releases and put it in place of the original steam_api.dll, the game will keep interacting with Steam as usual but it will consider all DLCs installed. Of course, you also need to download the actual DLC files from somewhere.
I’ve actually only tested it on Linux, so I’m curious to know if it works fine on Windows and MacOS.
If I made my research correctly, you won’t get banned for unlocking dlc in offline games. Online might be a problem though.
Although even the worst case shouldn’t be that bad. Games without VAC can only ban you in that specific game, only adding a red text on your profile page. It should not affect your Steam account in any other way.
Personally, I’ve used Cream API to unlock all dlcs in Cities Skylines, and had no major issues.