Wikipedia says "According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time."
Bernie certainly did not gain new public office in 2020. I therefore think Bernie's primary run does not demonstrate the window's shift on canceling student debt across America. Sure, he came closer in 2020 but no cigar.
So long as Bernie keeps his Senate seat, I agree that the Overton window in Vermont now includes cancelling student debt. We won't know for certain unless he is reelected there.
> the Overton window now includes completely eliminating student debt.
I'm referring to the fact that Bernie Sanders, who came close to being a major party candidate, ran on canceling student debt.