At least as much evidence as you have that they did.

Any impact protest voters might have had could be outweighed a thousandfold by nonvoters. Why call out protest voters unprompted as a cause of an election swing when their impact is a drop in an ocean?

Half the country didn't vote. If 1% of them leaned Biden they would still have had more impact than protest voters, and I feel like the percentage leaning Biden might have been slightly higher than 1%.

I feel like I see protest voters called out 300x as much as nonvoters and I can never understand why it's so disproportionate.

Idk if you're familiar with the phrase "proving a negative" or "burden of proof" but you should maybe try looking them up and learning a little bit about why you're wrong