Lack of built-in support HTTPS certificates and ARC headers for mailman2/3 in Plesk are known limitations of implementation for now (as well as supporting mailman3 only on two OSes).
We decided to start from this level of function because there is no degradation of functionality compared to mailman2; it allows customers to start switching to new OSes and mailman if they need to do that because of some reasons. Since 18.0.59 release, mailman3 on Debian11/12 OSes is officially supported by Plesk, it means if you have some issues with feature, you can contact with the Plesk Support team to get support.
Anyway, we have tasks to improve these aspect in the future, but currently, I can't say any ETA for that.