Apple removed third-party kernel extensions (and even so only removed the ability to install them without going into recovery and changing a scary box). There are still lots of .kext files in the /System folder for hardware drivers.

Yes, however that brings it closer to a micro-kernel model, if no one else is allowed to change the kernel as shipped from factory, having everything else done in userspace.

Additionally, if there is a userspace extension to an existing kernel extension, the userspace one will take precedence.