I really need to know that this is coming...
Mailchimp best practices clearly state that using multiple lists to manage the same group of people is NOT the right way.
When will MM be compatible with Mailchimp GROUPS?
This has become essential for MailChimp and people have been asking for this feature for years now.
Mailchimp changed its terms and conditions in May 2019. A lot of functionality, groups to tags and API were altered / removed. I have moved away as the Ninja plugin and other functions stopped working. 10yrs Mailchimp relationship gone.