A Good Potential Use Of Customer Lists
See the above screenshot from Gmail.
HMRC should know for a fact that my Self Assessment has been submitted already.
This would be the perfect use for Google Ads Customer (Audience) Lists.
They could have a ‘These emails have submitted a valid return already’ audience in Google ads to avoid showing irrelevant ads to people.
Though I’m not sure how long it would take to sync approx 10 million email addresses?