That’s not true. Many advertisers and even newsletters try to figure out which of their emails you actually read and when, so they can optimize subject and date

That’s not true. Many advertisers and even newsletters try to figure out which of their emails you actually read and when, so they can optimize subject and date/time for better effect - e.g. emoji in subjects on Sunday get better hits with person X, finance data on Thursday evening with person Y. They used to be able to tell where you were reading it from geoip, but google killed that by proxying all images through their servers as of a few years ago.