How do you know how many people who didn’t vote did so because of protest?

https://en.m.wikipedia.org/wiki/Uncommitted_National_Movement

Number’s more like 700k instead of 800k. There were likely more protesters not affiliated with this movement too, but this is the main organization.

magically disappeared

What makes you think it was magic and not a causal response to how both Biden and Harris dreadfully handled the race?