It didnt 20 years ago. That spec could have been changed and on routers added a default padding for the devices that didn't understand the new variable length.

It didnt 20 years ago. That spec could have been changed and on routers added a default padding for the devices that didn't understand the new variable length. 0.0.0.0.192.168.0.1 or whatever for the legacy ipv4 devices. Im sure there was all kinds of hemming and hawing about why it would be difficult to do on the current hardware of the time and none of that equipment even exists anymore. So instead we decided to reinvent the wheel and make it a major pain instead.