We do not have a Debian 9 running anywhere, but I would assume that update-rc.d still works, yes. AFAIK all it does is creating symlinks in the respective directories in /etc/rc?.d and there are probably still quite a few SysV init scripts lying around there, which have not yet been ported to SystemD.
If you are ready to make the move to SystemD however, there is an example for a SystemD service definition for ilServer in the ILIAS docs (search for ilServer):
https://docu.ilias.de/goto_docu_pg_75024_367.html
And if you are struggling to port the init script for the chat server, I might be able to look up our service definition for it (including some nifty parametrization if you need to run more than one instance of the chat server).