And, good question. It would be nice, though I'm sure they've buried it in their multi-page privacy doc somewhere.
EDIT: No, it's not opt-in. Reading failure on my part.
https://www.epubor.com/whispersync-for-kindle.html
"And "Whispersync for Books" is enabled on Kindle Fire, Kindle devices and apps by default."
https://smallbusiness.chron.com/amazon-whispernet-work-58992...
"Whispersync is on by default in all new Kindles, but you can turn off the option on individual devices if you have multiple readers attached to your account."
https://ebookfriendly.com/how-to-disable-data-collection-kin...
"How to disable data collection on your Kindle or Fire device"
That said, given that it provides a high value to the end user (I use it daily), I personally don't mind.