Thinking about this some more, there is a way we can do this.
Let's say you have two snapshots of the Leaderboard. The first snapshot is on February 2, 2017. The second snapshot is on May 18th, 2018.
By comparing these two snapshots you can identify all accounts who have played a game at some point between those dates, and remove those who have not.
So rather than expect the player to remember all accounts that they have used since Feburary 2, 2017, the TC could just publish that list, and ask players to refer to it and declare any of their accounts on that list.
This way both new players and existing players are treated the same.
Now as it happens, TC does have a list. It's not quite a full snapshot of the leaderboard, but it is a list of every player who has played a game since Feburary 2 2017, who is either
1. currently 624ELO or greater
2. who is currently below 624ELO, but was above 624ELO as of Feb 2, 2017.
There are 1,264 accounts on the list. For anyone interested here is a link:
This also allows us to look at (for example) the accounts who have made the best improvement in the last 470 days:
Many have asked the question - 'How big is the Active playing community'?
While we can't answer this question exactly without a full leaderboard export compared across two time periods, we can note that of the 1800 accounts currently ranked 624ELO or greater, only 1078 (60%) have played a game or more in the past 15 months.