Jellyfin does not specifically need x264 to function. I think an optfeature is more appropriate here, or adding an x264 USE-flag to the package that is default-on.
I'm pretty sure not having x264 in ffmpeg will break on-demand-transcoding on a default jellyfin installation, which for example will make you unable to play videos in the web client unless they are already encoded in a browser-playable format. My personal opinion is that this a core functionality that should work out-of-the-box and not something that should be easily broken by fiddling with use-flags. x264 is optional in the sense that you can (if you have the hardware) configure hardware encoding instead, but even in that case I would argue that being able to fall back on software encoding (for troubleshooting reasons for example) is more important than saving in on a dependency. On the other hand, if you have any specific situation where it's not feasible to install ffmpeg with x264, please tell us more about it and maybe we'll figure something out.
