The gradual roll out of this change started with a blog post[0] and included in-app notifications for the owners of impacted groups on GitLab.com.
If the group owner did not log in during the in-app notification period, they were then emailed (the email you received today) notifying that the group was impacted.
[0] - https://about.gitlab.com/blog/2022/03/24/efficient-free-tier...
Here's a question for Gitlab: "Why did you require me to give you an email address to sign up?"
The answer to that question means there is no explaining why they didn't use it first, and followed up with at least a couple updates along the way. This is exactly what the address exists on thier db for.