It sounds to me like Superhuman e-mail service is in permanent breach of the GDPR by collecting the reading behaviour and geolocation of every recipient of an email from one of their users. So that user can get a ‘message read’ signal, except it shows the user every time you opened a mail and your geolocation at that moment. Without the recipient’s knowledge, and thus without explicit consent, which is definitely needed for something like geotracking.
Also: switch off loading remote images in your e-mail client, so tracking pixels and other image based beacons won’t automatically load upon opening your mail.