When you open the "Description diff" tab on an issue, the app reads that issue's change history and the names and types of the fields in it from Jira, as you (Forge asUser). It can only read what your Jira account can read.
It shows versions of the Description and other text fields, and the display name of the person who made each change. Jira provides these names in the change history.
Nothing. The app has no storage scope, no cache, no database and no logs that contain issue text or names. If an unexpected error happens, the Forge log gets one line with the error type only (for example TypeError), without issue content.
Nothing. The app has no egress permissions and no remote back end. Everything runs on Atlassian's Forge platform.
Nothing. It has no scope that writes to Jira.
Display names and account IDs are only processed in memory to show who made each change. Account IDs aren't sent to the browser. Because nothing is stored, there's no personal data to delete or report later.
Use the support link on the Marketplace listing.