Alexim: I agree, and good luck, because I asked them almost the same things and they never did anything.
Ice_Mage: I got a reply yesterday. Worse than useless:
"The lack of the DLC items as well as the Eternal Arena Leaderboard is a known issue to us and a request to address those problems has been raised with an appropriate team."
The Eternal Arena DLC came out at the end of 2018. It's obvious since they haven't added leaderboards after half a decade, they never will. I'll try contacting the developer at some point, but they're perfectly content to sell a Legendary Edition that doesn't give you what you paid for, so I don't expect they'll care about an unobtainable achievement.
This support agent didn't even bother to fix the forum links on the store pages, or remove "Leaderboards" from the list of features for Banner Saga.
That's what I expected unfortunately.
Beyond the achievement, if it's clear that a feature doesn't exist and has never existed in that game, it should be removed from the description immediately. Instead, both the publisher and GOG are happy to provide false information.
In addition, the support agents seem to have no more room for action, or have been instructed to remain super vague under all circumstances, so by now their function is almost useless in most cases. After more than a month I'm still waiting for my Impaler key that was fraudulently removed from my library.