An original Sony Walkman had nothing in it that could ever require a "security update". It had no network access, no wireless connection, and as far as I know no microprocessor, nothing that could ever enable someone to hack into it - unless they had physical access and were skilled at soldering.
Even more so, the transistor radios from the 1950s were just simple circuits with as few as five transistors. Now these were radio receivers, so by modern standards you might worry about someone hacking into them. But with five transistors all soldered together on a simple circuit board, it would be difficult for an attacker to send a signal that would somehow reprogram the radio.
Maybe a lightning bolt though! That could cause a permanent denial of service.