MOSFET on-state characteristics
@Andy aka see here:
mhtml.alldatasheet.com/html-pdf2/50067/FAIRCHILD/2N7000/405/1/… , see
on-state drain current and on state drain-source voltage

The only occurrence of the term \$I_{D(ON)}\$ in the data sheet for the Fairchild 2N7000 is here: -

And the reason why most MOSFET data sheets don't list this value is because it can be easily extracted from the MOSFET forward characteristic seen here (with added red and blue lines): -

For \$V_{DS(ON)}\$ the only occurrence is here: -

And, if you look at the same MOSFET forward characteristic (as above) you'll see that ties in with the purple markings I've added to it below: -


As far as I know the degree of &quot;on-ness&quot; depends on the gate voltage,
so what does it mean to have a &quot;on&quot; state drain current Id on?

Correct and that is why most data sheets place emphasis on the forward characteristic curve because those (extremely) rarely mentioned parameters can be found in that graph. The forward characteristic graph tells you many many things and, you can easily interpolate as I have done.