Why is there still a /etc/init.d in debian 8?

Debian developers provide sysvinit script for fallback.

You can look it up at the Transition plan to systemd by default:

a) Providing a fallback boot entry for sysvinit when systemd is the default init in grub (#757298)

b) Developing a mechanism to warn on inittab configurations which are unsupported in systemd. (#761063)

c) Providing documentation on how to remain with sysvinit on upgrades and switch to sysvinit upon installation.

Also (I did not do any research if that is the case for Debian in particular) some systemd unit file are just wrappers around init scripts.