I, like many others, need to use unRAID on older Dell servers. After some digging I found someone that got it to work. They said they couldn't use the normal USB boot creation tool, but instead built the bootable flash drive in Pop!_OS Linux on an old laptop.
This appears to be from 11 months ago. I'm not entirely sure what steps would be followed on Linux to make this work. I did install Pop!_OS 20.04 and tried the `make_bootable_linux` script from 6.8.3-x86_64, but that didn't work