This is a myth. You can see in both 2018 and 2020, both of which saw Democrat victories, that NASDAQ composite actually went up within 24 hours. Same with the dow. You also see the nasdaq still up a week later in 2018. It was like 2% down in 2020 a week later, which is not meaningful for this argument especially given Covid is a confounding factor. I stopped doing research after this point for specific examples.

There is no meaningful trend or change that happens after elections. Some sector see some weird shifts that correct generally within days