This is the first I'm reading about this. I never got an alert or opt-in from Amazon. I was automatically opted in. If you use anything Amazon, Ring, Echo, Alexa, you're most likely opted in automatically.
Opting out.
What is Amazon Sidewalk, and how to opt out
Amazon’s wireless network, explained
www.tomsguide.com
Opting out.
Enable or Disable Amazon Sidewalk for Your Account - Amazon Customer Service
Use the Alexa app or Manage Your Content and Devices to turn Amazon Sidewalk on or off.
www.amazon.com