For the past week or so, Blogger (via firstname.lastname@example.org) had sent out email notifications to (likely) veteran Blogger users with the email body contents as follows...
This email address has a legacy Blogger account associated with it that hasn’t
logged in since 2007. In 60 days it will lose access to the account and
associated content; the data will be permanently deleted unless migrated to
the Google Account system at Legacy migration page.
There are no associated blogs with this account.
The Blogger team
You have received this mandatory email service announcement to update you about important changes to your Blogger product or account.
If you're one of the users who had received this email, this might mean you may have an old user account you've used to sign into, back in the days, to create an account or blog on Blogger.
If you are sure you have no associated Blogger blogs with this email, you can disregard the email notification.
If you have an old blog associated with this email of yours, you can continue to read on...
In 1999 to 2006 Blogger users only needs to create a username & password to sign in & start to blog.
During this, user's will also need to verify their email address attached to this created username at Blogger.
This email address (& data) is still kept at Blogger servers.
Then Google enforced the single Google Account sign in's throughout it services, which includes Blogger.
Users may retain the original email address signed into Blogger while the new Google Account was created. This email address can be a non gmail.com email address, as long as users can view their email inbox for verification purposes.
This practice to migrate the blog & user Google Account was excercised between 2009-2011 (roughly).
2017 looking forward, Blogger platform back end had a major overhaul. It looks like the platform is using new infrastructure & softwares, moving everything, slowly, away from the old software it was initially built on.
Google Account Deletion
Then 2023 Google announced that they will start to remove/delete/purge in-active Google Accounts that have not been active or signed in to within a 2 year period.
This deletion exercise will begin in Disember 2023 & Google also states the conditions on how to keep an account active when this exercise will be enforced.
Why This Blog Migration Email?
Putting 2 & 2 together, it is likely Blogger platform is running an exercise to remind users to update their Blogger sign ins account & migrate their blog using the migration tool (link in the email body).
By doing so, will attach this old email address credentials, with the blog on Blogger platform.
In addition, by signing back in, this will reset the 2 year in-activity counter to prevent the account to be deleted by Google systems.
This is probably the reasons why in the Blogger email body states "...in 60 days, the blog will be deleted if it is not migrated.." -- perhaps inline with Google's account deletion thinking forward.
Why You Should Worry?
If you care about your blog contents, created circa 2006 to 2011 & also the images uploaded with it, you can continue to migrate your blog & account using the Migration Tool link in the email body.
If you've already assigned several Admins for this blog, and the Admins are active, meaning the Google Account is signed in regularly, then there's nothing for you to worry about.
It is a good practice to create a Backup of your blog contents using Blogger > Settings > Backup Content feature or Backup your Google Account uploaded assets by using Google Takeout https://takeout.google.com .
Ensure you are signed out of all Google Accounts at your web browser & only sign in with the Google Account attached to this blog in order to download the correct blog Backup.
Based on our findings, if you have multiple Admins assigned for the blog, and the other Admin(s) Google Account is still active, the blog will remain online.
Things you need to take note, if the old account had published contents at this blog, and this old account is deleted, the Profile name might change, possibly to "Unknown" and the images uploaded by this account will be empty/missing.
The contents will remain published so other Admin(s) can edit to re-upload replacement images, if needed.
Hope it Helps!