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...