Load untrusted kernel extension in macOS Mojave
I wrote the developer and received the following reply (I hope he doesn't mind me posting it here):


  I would prefer not to have the limitation of disabling SIP to install Volta's kernel driver. But Apple has rejected my request to sign the driver (understandably) as it makes low level changes to the CPU Machine Specific Registers.
  
  You can read about MSRs in Intels architecture documentation (Chapter 35): https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3c-part-3-manual.pdf
  
  Why is the kernel driver needed? In order to execute WRMSR and RDMSR instructions kernel level permissions are required.
  
  It is possible to partially disable SIP by only allowing unsigned kernel drivers to be installed, but maintaining other SIP protections.
  
  Unsigned drivers used to be allowed until recent macOS versions; as long as you are careful with what you install on your Mac you should be fine.


I replied asking if Apple also had veto power over kexts even if they were not distributed in the App Store. His reply:


  Correct, regardless of distribution method.
  
  https://developer.apple.com/library/archive/documentation/Security/Conceptual/System_Integrity_Protection_Guide/KernelExtensions/KernelExtensions.html
  
  "You can request a Developer ID Certificate for signing kexts by visiting https://developer.apple.com/contact/kext and filling out the required details."
  
  My request was rejected, I even referenced the 'Intel Power Gadget' in my request as it uses similar MSR instructions as Volta.
  
  I don't think Apple likes the idea of users controlling their hardware. Especially when undervolting can cause stability issues.


Therefore, it looks like if you want to install a kext not blessed by Apple, you have no choice but to disable SIP. Looks like Apple, through some convoluted reasoning, thinks I'll be safer if, in order to run code they don't approve of, I also have to open a gaping hole in my computer's security in the process.