I already received a response to the original request (FB19631390), they are not planning to make improvements on that side. So I created a new request: FB19761610 (Package Installer: Ability to install and activate System Extensions) I couldn't find a category specific to the Installer, so I selected the System Extensions Framework category.

Thanks Quinn! So it seems to me that app extensions are unlikely to be supported outside of the Mac App Store. And system extensions are more wide reaching, so they require the user's understanding of that. I suppose the best bet is to hope Apple will consider the feedback and improve the system extension installation UX. I just had an idea, maybe the system extension installation could be part of the .pkg installer, because it can already install some other system level components. I will add this idea as a comment in that enhancement request. Or should I file a separate one?