The resolution does mention there will be documentation on switching to sysv after an installation, so I assume this option will have to be officially supported; it sounds to me as though systemd is simply a default, not a requirement.
Yes, it will be officially supported as systemd is simply a default, not a requirement, as defined by previous CTTE deliberations and as implemented even before them by the Debian systemd team, which has gone to great extents to make sure you can happily switch back and forth with almost no issues.