What? Literally every electoral vote is unpledged.
https://upload.wikimedia.org/wikipedia/commons/1/1d/Faithless_elector_states.svg
List of states with laws against faithless electors (i.e. they are not only pledged but -legally bound- to vote for the winner of their respective state.
There is no electoral college equivilent of superdelegates who are absolutely unbound, can vote for whoever they want regardless of the vote outcome--the electoral votes are the equivilent of the pledged delegates.
I'm aware that some states have laws on the books, but who knows how such laws would be enforced or if they would hold up in Court (I'm guessing they wouldn't).
But if it makes you feel better, I can revise my statement to "Around half of the electoral votes are unpledged." It remains a legitimate point that Sanders staying in after California is equivalent to Sanders losing to Trump in November but refusing to concede because he wants to try to convince the electors (for argument's sake, we'll say only in the grey states on your map) to pick him instead. Legally allowed, but total horsesh**t.