The alt text of the leading images were displayed as preview
content in inbox by email clients like gmail. Since the leading
images were used mostly for decoration this made the preview
content gibberish. It's fine to set the alt attributes to empty
from accessibility point of view since the old alt attributes
did't added any meaningful information.
Hopefully this does a better job of spurring people to action, and also
suggests a self-service fix if they don't (i.e. contacting the person that
invited them).