They did have that capability (see https://wiki.mozilla.org/Firefox/Go_Faster/System_Add-ons/Pr... ), at least the linked repository had commits in 2016. I can't tell from a 30 second search why that no longer works, just that it is a replacement of a similar previous capability.
This started before my time at Mozilla, but I think several different systems grew out of the "go faster" initiative. Of them I believe the fastest and most flexible means of shipping changes is Normandy, which is primarily used for our shield studies. Like the blog post says, we need to revisit this.
