Earlier quoted context omitted.
Probably no reason, they said forwards will remain free. Hopefully that includes aliases as well, I've got a wildcard alias that is awesome to use. How do you know they're adding Sieve support? The email just says "Better anti-spam" The email also says "Stronger email server security" which I hope means they are fixing their MFA scheme - I've sent an email to their new support address to confirm this.
https://docs.gandi.net/en/gandimail/sieve/getting_started.ht...
In my Gandi portal I don't yet have an option to switch to "new" GandiMail. I figure their docs aren't updated yet either. Your legacy mailbox probably already supports Sieve.