Why do we keep describing apps as "missing" from an OS? Surely we're not implying that an OS should ship with all these different tools, right?
https://duckduckgo.com/?q=%22the+missing%22+*+from+OSX (not the best search but it shows a few. In my mind I seem to recall having read this a lot though)
Also, the app looks great. I'm using RESTed right now and looks like this does all the things I wish RESTed would do.
kernel
kernel + default userland
Kernel + default userland + kernel vendor add-ons
kernel + default userland + kernel vendor add-ons + ecosystem software
Also, since linux gained traction OS can frequently be interchanged with distribution of a kernel + userland + packaging tools + software repository + ecosystem, in various ways, and as above.
Not sure why you decided that OSX means strictly:
kernel + default userland
I was just focused on the "missing" term, to me that implies something that was once there or is notable in it's absence. However, as with many of the things that describe themselves in this way, that is not the case.
In this particular case, HTTP clients for the OSX ecosystem are not missing, they're just not that great.
In general I just dislike the wording.