newsletter page shows only a form for entering the mail, even if the user clicks on the email to unsubscribe. The user subscribes to and confirm his address. but on the page do not show him messages but only a form for entering a mail button for confirmation.
The 404 plugin should not remove the URL parameters. Probably it has a configuration. Once upon a time it was a best SEO practice but now with the canonical url metatag it is no more required!