You only need to be sent an "opt in" email if the sender doesn't have a GDPR compliant audit trail back to your original request. However with the confusion over the whole thing some people are requiring opt-in anyway.

Another thing is that I am getting requests from people who hold no personal data about me. My working email address is jobtitle.businessname and as long as they only have my job title and not my name on record it isn't covered by GDPR at all.