I decided to extract the immutablekernel for Catalina 10.15.3.
I had to add a FAT header but it extracts just fine.
Thanks to this we have the kernel and kexts for the immutablekernel.
The info dictionary and lists are also included.
Needs further study if it contains extras compared to the prelinkedkernel.
Maybe we can find out what those .im4m files (for example immutablekernel.x589iclydev.im4m) are thanks to this.