As far as I know, it's impossible to delete accounts. But there are many accounts that have been deleted.
Start from user ID 1, change it to bigger ones and see how many accounts have been deleted.
I am curious to know who had been the owner of the account with user ID "2".  This account is even older than Satoshi's account and it is already removed.
Now there are many missed user IDs. Why have they been deleted?