Update 4/15/2024: Please Note: No timeline has been communicated on support for MySQL 5.x. When it is, Cloud SQL will also publish a blog post and update the following page to reflect the version support timeline.
https://cloud.google.com/sql/docs/db-versions
Please refer to Major version deprecation plan You can expect notification later this year around EOL for MySQL 5.6 and 5.7 on Cloud SQL