The state legislatures never lost the power to appoint electors as they wish.
Maine (only since enacting a state law in 1969) and Nebraska (only since enacting a state law in 1992) have awarded one electoral vote to the winner of each congressional district, and two electoral votes statewide.
The normal way of changing the method of electing the President continues to be by changes in state law.
Historically, major changes in the method of electing the President have come about by state legislative action. For example, the people had no vote for President in most states in the nation’s first election in 1789. However, now, as a result of changes in the state laws governing the appointment of presidential electors, the people have the right to vote for presidential electors in 100% of the states.
In 1789, only 3 states used the winner-take-all method (awarding all of a state’s electoral vote to the candidate who gets the most votes in the state). However, as a result of changes in state laws, the winner-take-all method is now currently used by 48 of the 50 states.
In 1789, it was necessary to own a substantial amount of property in order to vote; however, as a result of changes in state laws, there are now no property requirements for voting in any state.
In other words, neither of the two most important features of the current system of electing the President (namely, that the voters may vote and the winner-take-all method) are in the U.S. Constitution. Neither was the choice of the Founders when they went back to their states to organize the nation’s first presidential election.
The normal process of effecting change in the method of electing the President is specified in the U.S. Constitution, namely action by the state legislatures. This is how the current system was created, and this is the built-in method that the Constitution provides for making changes.