Wrapping an LSB init script with systemd-sysv-generator(8) is not an ideal way to run a system service. In particular, systemd-sysv-generator can't tell whether an init script is meant to be the equivalent of systemd Type=forking (like /etc/init.d/dbus) or the equivalent of systemd Type=oneshot (like /etc/init.d/sudo), so it has no choice but to create a weird mixture of the two which doesn't reliably detect errors.
I suspect that a future version of systemd will deprecate systemd-sysv-generator and stop installing it by default, although I don't think any such plans have been announced yet.
Here's a native systemd unit equivalent of the LSB init script, which is much shorter and simpler than the init script itself: https://git.pseudorandom.co.uk/smcv/ikiwiki-hosting.git/shortlog/refs/heads/native-systemd-unit
I haven't removed the LSB init script, so users of non-systemd init can continue to use it as-is.
--smcv
The service file looks ok.
I'm a bit unsure about the two ikiwiki-hosting.conf that don't get moved to the service file. If a user has changed those, they will have to notice the conffile change and follow through to edit the service file based on comments in the conffile.
Also, ikisite uses those configs, so we lose DRY and the user has to remember to change a config in two places if they change it later. (To be fair, the postinst also creates a user based on those configs so changing them is already involved and probably noone actually does.)