A no-login comparison
Both relationship lists are read locally. CheckFollowBack does not receive the ZIP, the visible username groups, or a downloaded CSV.
What the browser may retain
The current result lives in page memory. A normalized follower baseline may be stored in IndexedDB for later history, even though the core follow-back question needs only the current export.
What optional services can receive
Production logs may contain ordinary request metadata. Privacy-limited analytics may receive event names and broad size bands. A sponsored provider receives a request only after the user opens that optional enhancement.
No exact list or archive content is an analytics or offer parameter.
Your controls
Delete local data removes the baseline; reload clears current rows; downloads must be removed separately. Contact [email protected] without attaching the relationship files.