There's nearly 2,000 users on the RuneTrack forums. Removing players previous usernames individually is just too much work for one person (Sword Kill11 runs the website alone) to manage free of cost.
To allow players to manually erase their previous usernames would mean creating user accounts for RuneTrack, which would also mean that anyone can pretend to be someone they're not. Every player that demands their previous usernames erased would require in-game proof that they are indeed that RuneScape player (pictures aren't that hard to edit).
But yeah, it would be nice to be able to erase old usernames. Only way around this would be to create something that; if you've had a username for x-amount of months, then your previous username(s) gets automatically deleted.
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