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...
I think there is a glitch in your mail or something else is going wrong. I'm currently not in any groups and still got an e-mail telling me that my top level group (starting with 5060) has reached the 5 members limit. Searching for the group also doesn't yield any results whatsoever.