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.