The domains were registered using Google Domains, but they were set to automatically renew, so Google's message was confusing. Google realized that it was a mistake and sent another message:
"An automated email was sent out to you earlier in error with the subject 'Your domain registration is expiring'. Please discard the erroneous message. Since your domain is set to auto-renew, your domain will be renewed on the expiration date as expected. We apologize for any confusion this may have caused."
Unfortunately for Google, the message was sent to 1664 people and all the email addresses were added to the "to" field, so everyone can see them. Whoops!
Update: Google sent a new message. "Yesterday, we sent an email to select Google Domains customers in which all recipients' email addresses were visible in the 'To' field. We sincerely apologize, and we'll be evaluating our customer email processes to prevent an accident like this from happening again. Thank you for your understanding."
