This is for compatibility with GNU readobj. "--arm-attributes" option is left as a hidden alias due to large number of tests using it.
Looks good to me. GNU readelf --arch-specific also handles MIPS .MIPS.abiflags, PowerPC specific tags, RISC-V specific tags, etc, which llvm-readobj does not support now but these targets are supported by LLVM. CC some people for more opinions.