Shamelessly stolen from another forum site:
Just found out Plex has updated its Privacy Policy page and added a new page to opt-out trackings.
https://www.plex.tv/about/privacy-legal/
https://www.plex.tv/vendors-us/
Up until now all your playback data may be involuntarily sent to ad agencies and companies such as Amazon to silently influence and target your buyings. Now we can opt out everything and keep your watch habit private and treat your plex server like never existed.
EDIT: now you can also easily access privacy settings and delete all already uploaded watch history, and stop syncing watch states, rating etc. Remember to set all to private if privacy is important to you.
https://prnt.sc/hj6ZNNh31nl3
https://prnt.sc/0DGbRGLI97_G
https://prnt.sc/er3dYwJnut9j
EDIT2: also double confirm "Send playback data to Plex" is unchecked.
https://www.plex.tv/about/privacy-legal/#you-have-the-right-to-limit-what-activity-data-is-sent-from-your-plex-media-server