Now that systemd the preferred way to start system services in CentOS 7, can anyone tell me in which release the init system will be completely removed? i.e no support for init scripts whatsoever?
systemd provides the compatibility with traditional init scripts.
To quote from Compatibility with SysV:
systemd provides a fair degree of compatibility with the behavior exposed by the SysV init system as implemented by many distributions. Compatibility is provided both for the user experience and the SysV scripting APIs. However, there are some areas where compatibility is limited due to technical reasons or design decisions of systemd and the distributions.
So the answer to your question would be: "When compatibility with SysV is removed from systemd."
But note that the document also says:
Note that there are some areas where systemd currently provides a certain amount of compatibility where we expect this compatibility to be removed eventually.