Generally when we send confirmation emails to addresses the user has
not already proven that they control, we want to avoid including in
the email any user-controlled data. Doing so makes it hard for
malicious actors to use the feature to send spam, since they won't
have a way to include the URL for their malicious website in our
emails.
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.