There is no specific last used date field. You could look in GL_DetailPosting at the last Postingdate for a GL account to get some idea.

I would recommend you don't delete any accounts that have been used. You can make them inactive so they won't always be in the drop downs.

If the account has any balance or any transaction history in the system, it should not let you delete the account.

There is a third option for account status called delete. This is the same as setting it to inactivate, but it will automatically delete during year end processing the first time there is no longer any balance or transaction history associated with the account.