Also, if the transaction is related to a business document, such as an Invoice, Bill, Shipment or Inventory Build, then that document is also permanently saved. Reversing a transaction on one of the business documents will cause that business document to be voided. You can therefore always view the documents that were related to a reversed transaction.
Whenever a transaction is posted, the accounting transaction and related transaction details are permanent. You can always reverse a transaction -- which is simply another transaction with the debit and credit postings swapped for the affected accounts -- but you may never delete it.