Here is the quote from the 2018 California Smog Check Reference Guide.
"Customized Exhaust before last required after-treatment component – Header5 Heat Risers, Oversized pipes, etc." Require a CARB EO#.
"before last required after-treatment component" that would be the Cat efficiency (last O2) sensor.
Here is the way I interpret this:
Of course if it has a CARB EO# it is legal.
However in this case, the Header, fittings for both O2 Sensors, and CAT are all one assembly.
For that reason all of that requires a EO#.
If you were to even replace the CAT with a legal one (if it exists) then the header pipe is modified and that would be illegal.
So if there is such a thing as a legal aftermarket CAT then it would include the complete assy.
This is why there not many of these things CARB approved, it is too expensive to make and get approved.
Please remember I am only referring to the state of California.
Guys, I am not the smog police (any more) and never was after work.
I don't care what you do to your own car.
Dave