Article II, section 1: Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress:
The 12th Amendment recognized the role of political parties and changed the idea of the VP being the man who came in 2nd in the Electoral College vote, and mandated separate ballots for President and VP
The 20th Amendment changed the dates of Presidential inauguration and the beginning of the new Congress. It also deals with what happens if the President-elect should die before taking office.
Neither amendment changes Art. II Sec. 1.
The State of Illinois already has its laws in effect regarding the manner in which electors are appointed.
That is not the issue
The issue is whether Donald J. Trump can be kept off the state ballot by a state legislative action and the answer is a resounding No!
Trump can only be denied ballot access by proving he does not meet the requirements for candidacy as set forth in the US Constitution. The State of Illinois has no control over the US Constitution’s eligibility criteria for President.