Obviously, your reading history cannot be seen by anyone but you, and it is important to emphasize that Reading Queue does not operate any database server of its own.
Your queue and archive are backed up and synced across all your iPhone, iPad and Mac devices where Reading Queue is installed. All your data is stored ENCRYPTED in your PRIVATE iCloud storage, which means that ONLY YOU and NO ONE ELSE can access them, not even Apple or the developer of this app. You can turn this feature off and erase everything from your private iCloud space any time.
Reading Queue for iOS uses Google Analytics for Firebase to track basic app usage: whether onboarding steps are completed, when you tap the menu, swipe a page... these kind of things. But nothing personal like what you read or which sites you visited.
Finally, the Reading Queue browser extensions for Safari, Firefox and Chrome do not use or inject any kind of tracking script. They just communicate securely with Apple's CloudKit web services.