This post (Why were empty accounts allowed to be on the blockchain?) discusses the Ddos attack from fall 2016. It mentions that nearly 20,000,000 account were created and then immediately deleted causing a huge bloat in the state trie. Does anyone know during which blocks those Ddos attacks were created?
If one were to ever try to create an index of accounts, that index would become unnecassarily bogged down. If one knew during which blocks they were created, one could simply skip these accounts given that they would be deleted soon thereafter by the hard fork.