Subject lines have always been the best way and most emails could be nothing but subject line if blank bodies were allowed.

But because subject lines are more work and people who love sending email tend to love it because it is very low effort, the venn of email senders and those who write subject lines is small.

even in the best scenarios, a well organized group of people is not using all the features of email "properly" to say nothing of groups that are supposed to have the general public.