The Prosody source tree itself does not contain init scripts or packaging things related to specific platforms. Also none of the distributions that a member of the Prosody team maintains packages for use systemd, and I (sadly) don't directly know the packagers of any platforms that do.
All this said though, I have updated our documentation for packagers to point people to this issue, so if anyone is researching packaging Prosody for a systemd platform they should end up here. See http://prosody.im/doc/packagers#init_script .
Feel free to update this issue with further revisions and your findings if you manage to test there.
Here's what I believe to be a more "pure" systemd service file, that does not use prosodyctl:
Description=Prosody XMPP Server
# Do the things normally done during daemonization here instead
# Needs daemonize = false in prosody.cfg.lua
ExecReload=/bin/kill -HUP $MAINPID
Honestly, wouldn't it be better to use mod_systemd and its watchdog features?