How much legacy code do I need to understand and fix before I add Software Archeologist to my LinkedIn?

It is called "maintenance" and has to be done too.

All of it

@Oktokolo Hmm, Software Mechanic it is then

@Monk
Most mechanics sadly don't refactor the stuff they touch.
But yes, it would surely be a better world if they did...

"Hmm, this model has a known design flaw. Let me quickly CNC a better version of that part, which will likely break under stress after a few thousand miles, so you don't have to come back for it next year."

@Oktokolo The same can be said about programmers, unfortunately.

@Monk
Yeah, most only like working on the newest shit of the day.
But that isn't sustainable. Soon, IT has to mature because security becomes safety when modern life more and more depends on software to be reliable but still needs to be interconnected.

"Power down for weeks because some kid knows how to reverse shit" isn't an outcome to desire. And connection to the internet will inevitably happen. So software will need to be resilient and free of some of the most common bug classes today.
Just translating everything into less booby-trapped languages isn't enough - maintenance and continuous improvement has to happen too.