I have published a new Mailman 3 plugin at https://mail.aegee.org/cgit/mailman_sieve/tree/ . It generates for each mailing list a Sieve script. When the Sieve script is evaluated during the SMTP dialog, it would reject some emails, which otherwise would be bounced by Mailman. Less bounces means less backscatters and better IP reputation.
For the record, I do the evaluation of the Sieve scripts during the SMTP dialog with https://mail.aegee.org/cgit/aegee-milter . Since the latter is not documented yet, you cannot use it. The mailman_sieve plugin is not useful, unless you have software to evaluate the Sieve Scripts during the SMTP dialog.
This is a very first version, which does what it is supposed to do. Some optimizations are possible, like not including the subscriber addresses in the Sieve scripts for announce-only mailing lists. I am not good at writing setuptools configuration. Neither do I understand the mailman system very good right now, creating possibly shorter scripts. But the plugin is there to keep by IP reputation good!
My reading of the mm3 source code is that there is no event triggered, when the mailing list configuration changes, like adding a new moderator, or switching the announce-only status. In the lack of such event triggers, the Sieve scripts are not updated.
When the code improves, the plugin could be published in PyPi or in mailman/contrib .