They were added to the game in May 2010, according to the report, shortly after which Nexon began messing with the drop rates so more powerful Cubes would appear less often.
It continued changing the odds until March 2021, in some cases ensuring that particularly high-demand Cubes didn't drop at all, but did not notify players of the changes—in fact, in August 2011 Nexon released a statement saying that no changes had been made.
IGN:
Nexon in particular has become known for its aggressive monetization practices. Last year, IGN reported on MapleStory's controversial "New Age" update, which made it more difficult for players in certain regions to farm a key currency while the price of some items were inflated. The update led to large-scale protests throughout the community.
An intense debate ignited November 13, when The Game Awards host Geoff Keighley announced the nominees for this year’s trophy ceremony. ... when the nominees were announced on November 13, many folks online pointed out that Dave the Diver wasn’t, in their opinion, an indie game, despite looking like one.
...
So yeah, Dave the Diver. That game is made by a group named Mintrocket, [so it’s] a smaller game from a smaller group, but [Mintrocket is] part of Nexon. They’re employees of Nexon, which is a very large publisher. So, I think it’s a fair debate and discussion about is that game truly independent or is it not. You can argue it either way. It’s independent in spirit