The IPFire team is quite small and all are volunteers who also have day jobs they have to do to pay the bills.
Main focus is on the core packages and addons get updated as someone is able to get round to them or a forum user flags up that a package is out of date or there are CVE’s flagged up.
From your notification I have noted that the latest stable version has fixes for various CVE’s so I will do a package update for nginx.
This is not installed by default as the nginx documents indicate. It’s purouse also seems to be to slice large files such as video files that are being downloaded via the nginx http server.
nginx’s use in IPFire seems to be as a reverse proxy server or mail proxy server and not as a webserver to the internet.
I don’t use nginx and have no idea on the suitability or not of that slice module on a firewall system.
If you believe that this is a required functionality on the firewall then you can of course submit a patch into the development mailing list that adds that module, together with the argumentation for its benefit etc into the patch commit statement.