Ian, I find that I'm not able to answer Simon's question with regard to Proposal D.
Imagine that we have a program that has compile time support for systemd and for other mechanisms. It provides enhanced functionality when built against systemd, but when so built, it cannot run without systemd. It's packaged that way in Debian. Someone files a bug with a patch that changes the compilation option to support the non-systemd bug, removing the enhanced systemd functionality. What does proposal D say about this? Is the package RC buggy under proposal D until this patch is applied? Does the maintainer have the option to retain the enhanced functionality?