Not only was it the plan, it was written in Article I of the Constitution.
Has that portion been altered or voided by an amendment?
As the War of 1812 demonstrated, it was a bad plan. Subsequent to that war, the state militias gradually disappeared as the federal government created a standing army.
There is no Illinois National guard?
Sec. 311. - Militia: composition and classes
The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
The classes of the militia are -
the organized militia, which consists of the National Guard and the Naval Militia; and
(2)
the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia
Like I said, the plan. The intention, if you will.
"Has that portion been altered or voided by an amendment?"
Nope. It's just not used anymore as we do not have state militias. The states don't appoint officers either, and that's in Article I also.
"There is no Illinois National guard?"
There is now. And the Guard is bought and paid for by the U.S. Government, not the State of Illinois, and are under the President's orders, if he so chooses. Unless they all go AWOL, I wouldn't consider them to be "securing a free Illinois".
"The militia of the United States ..."
Yes, the militia of the United States is so described. This is not the well regulated state militia. Two different animals.
10 USC 311 describes the organization that replaced the well regulated state militia described in The Militia Act of 1792 (an attempt to standardize the independent and individual state militias).