What is the secondary CPU Usage Graph line in the Windows 10 Task Manager

That looks like the kernel time graph. Right-click the graph and see if this option is ticked.

Kernel time has been available for a while but it was more hidden.

Seems it was available from at least Windows XP : https://blog.codinghorror.com/everything-you-always-wanted-to-know-about-task-manager-but-were-afraid-to-ask/

The light blue line represents the total amount of time your CPU is spending on tasks. The dark blue line represents what percentage of that time involves the kernel.

What's the difference?

If an application maxes out the %CPU load but the kernel time is still low, then the system still feels responsive and snappy. This is because the kernel's primary job is to schedule CPU time for processes and it can preempt one process to run another when necessary.

Only when the kernel pegs the CPU at 100% does your computer feel slow and sluggish. That's why the option is there to select because it gives a more realistic view from a performance perspective than the regular graph does.