I don't doubt that codicils (even wills) have disappeared but I wouldn't have thought that beneficiaries had access to such documents unless the benefactor hasn't properly secured them.
These were supposedly stored securely in a safe at the attorney's office, but somehow got "lost in transit" between the attorney and the accountant in the adjoining office.
As regards to executors failing to execute them, isn't that a crime?
Technically, but proving it is not always easy and is always expensive, frequently more expensive than the potential gain.
Having been burned twice, I file my will with the County Clerk, and any subsequent changes or additions become
amendments to and part of the original filing. Besides that, filing the will with the county clerk saves the executor a step when the will is probated.