Separator is a very generic component you might want to use more than once in a panel and should be promoted as one of the "default" items.
Simon Steinbeiss referenced this bugreport in commit 9c2416cdb240ae174f14d6f03f2ebf44ebab4f8c Move "Separator" to the top of the "Add New Items..." menu (Bug #14086) https://git.xfce.org/xfce/xfce4-panel/commit?id=9c2416cdb240ae174f14d6f03f2ebf44ebab4f8c
After reading the code a second time it turned out to be much easier than anticipated to implement this.