Yeah, that would be it - not implimented.
It seems that there are a few times in sponge forums where it has been noted that LaunchProjectileEvent has not been implemented yet.
If you have the skills to do so, making a pull request to impliment it would make a lot of people happy, or look for a PR that was made long ago to impliment it but which failed or has failed to be added for simple reasons, like not formatted correctly or missing a response from the devs to change a line of code and post in it for a status update / snag that and replicate your own PR, fixed up and ready to be go.
Otherwise, youll need to write your own mixin-plugin to add the hook for the event, or do reflection work on the server jars.
It does seem hard to believe that this far into the project, one of the core, often happening processess of the game that has existed years before sponge was concieved does not have its event being thrown - its not like its a really new feature, or a super-system being designed from scratch, and that it has managed to be overlooked for this long, so its likely a matter of there being PRs for it that were made but fell short at the last second, or failed to compile, or failed to get approved, and yet triggers a vague "I'm sure I remember that being done" response from everyone on the dev team gloss-over