Final answer:
The legacy system's issue with ICMP packets larger than 65,535 bytes suggests it is experiencing a 'Ping of death' attack, which is caused by oversized packets designed to disrupt the system.
Step-by-step explanation:
A system that is no longer responding due to being flooded with ICMP packets larger than 65,535 bytes is most likely the result of a 'Ping of death' attack.
In this type of attack, the perpetrator sends malformed or oversized packets to crash, freeze, or reboot the target machine.