The readers reactions are stored as custom variable within the page database itself. A temporary storage is also applied within the session, which is just used to check if the "user has already been voted" (and show them respectively on the frontend panel). This has the disadvantage that the user can vote again if he restarts the browser, of course. But I decided this way to avoid GDPR conform questions, and in fact that this plugin has been written in less than 2 hours. A better storage may gets added in a future release.
Since today a new version is also already available, which allows you to adapt the Panel title, configure the used hook (where the panel gets applied) and which fixes the issue, that this panel gets shown on custom 404 error pages.