"Legacy" are the features of AG2 that are rewarded in AG3, when it is removed. For instance, those achievements you've gained from your ranks are your Legacy achievements, because those are no longer available to newcomers.
It's actually more because anyone with the Gold Prince rank will also have access to the 10000 AP armatars.
Last time I checked, it was seriously the top 100 people who had 10,000AP. The administrators couldn't have picked a better time to introduce achievements.