Nebraska and Maine both alot their electoral votes according to the system the ballot initiative proposes for California.
In fact the several states can chose to allocate their electoral votes any way they want: if the legislature of a state wanted to, it could decide to select the electors itself (though the voters would bounce out the party that voted the measure in in the next state legislative elections).
Of course in states like California with initiative and referendum, each election day, the electorate becomes the legislature, and likewise can decide to adopt any measure for selecting presidential electors they want, be it the Nebraska/Maine system, or a requirement that electors not formally committed to any candidate campaign in districts and be personally elected as electors, or any other procedure they like, including the ‘let’s partially disenfanchise ourselves’ system of picking a slate committed to the winner of the national popular vote that the ‘RATs are pushing.
I rather like the proposal. Obviously if it is adopted it tilts toward the GOP, since the demonRATs can’t count on the LA basin and the Bay area to deliver them CA’s 55 electoral votes as a block.
Same applies elsewhere of course.
It might lessen the impact of TV media in LA, SF, NY and other big cities 'cause they would probably want to spend more time in flyover territory.
This system lasted in at least a few states well into the 1800's. The winner-take-all system is not required by the Constitution, which leaves the means for "appointing" electors up to the individual States.