“To the extent that commentators acknowledge the issue, the conventional view is that states have absolute discretion to appoint their delegates in any manner they see fit...
...Disagreeing with this conventional wisdom, this Article concludes that the states lack the power to appoint their presidential electors on the basis of votes of citizens outside the state’s jurisdiction and, therefore, states are without authority to adopt the NPVC.”
https://digitalcommons.law.byu.edu/cgi/viewcontent.cgi?article=2686&context=lawreview
Norman Williams makes an interesting argument - but I don’t think it would actually hold up.
Article II, Section 1 certainly does give the state legislatures the right to appoint electors in whatever manner they want to do so.
“Each State shall appoint, in such Manner as the Legislature thereof may direct”
His argument against this relies, in part, on the decision made in the “U.S. Term Limits, Inc. v. Thornton” case in 1995. It decided that the states could not impose term limits on their members of Congress.
The argument in favor of allowing the states to imply these limits came from Article I, Section 4, specifically the part that states that the states may set the “Times, Places and Manner” of holding elections for Congress.
The (successful) argument against was that this imposed a new qualification requirement on members of Congress beyond those specified in the Constitution. And since this encroached on Federal authority, it should not be allowed.
Another part of the argument comes from Article I, Section 10, which reads “No State shall, without the Consent of Congress ... enter into any Agreement or Compact with another State”. However, in “Virginia vs Tennessee” (1893) it was decided that this only applied in cases where the compact between the states eroded the powers of the Federal government.
The approach neither places additional qualifications on elected officials, nor does it erode the powers of the Federal government.