/sbin/init points to systemd hence systemd is used. Can we safely change it into the old SysV?
According to http://without-systemd.org/ Debian Jessie loses polkit. It's not particularly practical to run a desktop without a privilege escalation mechanism e.g. for installing updates. (As the name PolicyKit suggests, the idea is is to allow different policies - e.g. for different groups of users, or for different Linux distributions).
I'm not saying Debian was wrong to keep it in for another release, but the problem when you take advantage of the mix&match approach, is it's hard to know what is actually expected to work.
I'm not sure if this counts as explicitly deprecating it, but this sounds like "you better already know what you're doing".
Be advised that some packages may have degraded behavior or may be lacking features under a non-default init system.
Server systems are much less likely to see problems in this context.
Because systemd came later, it has a mechanism to run old SysV init scripts for forward compatibility.
Packages designed for a systemd-based system may only ship a systemd-based "service" script and not a legacy "init.d" script.
I don't think SysV has a way to read systemd service files. Therefore, some services might not start properly when using the old init system.