UEFI is a ubiquitous firmware environment that is similar to an OS environment. However, downstream projects cannot differentiate UEFI toolchains from those of the none os constraint. UEFI has a specific ABI (i.e. PE/COFF), whereas the none os constraint generally matches a SYSV ABI (i.e. ELF).

bazelbuild/rules_rust is one project where UEFI target support is blocked by this. (bazelbuild/rules_rust#2142)