Please note, Plesk has supported the version of Mailman provided by OS vendor by default.
The Mailman 3 is shipped by default on Ubuntu 18 and Debian 10.
Please keep voting if this request is still actual for you.
I would really like to see Mailman upgraded please.
Thank you for your input! We will consider this functionality in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.