Remove drop shadow from Markup Toolbar shape in Preview PDF
Found the answer on Ask Different when looking for an answer after not finding one here.
Select your shape, then go to the Shape Style drop down menu and choose the bottom most option for line style. That turns on/off the drop shadow for your shape. So simple!
Here's a link to the Stack Exchange Ask Different page where I found this solution. I am not at all affiliated with them and never saw that page before a few minutes ago.
I am running Sonoma 14.6.1 on an MacBook Pro M1 2021.
MacBook Pro 16″, macOS 14.6