Input Needed: Teams for Pull Request Subscriptions - #55 by jh7370 is related.
I think there are two different goals. One is to list just “core” backend files, which our current .github/CODEOWNERS patterns cover. The other is to enumerate nearly everything related to a target.
For example, a developer interested in AArch64 may not want to know changes to llvm/test/tools/llvm-objdump/ELF/AArch64/, bolt/lib/Target/AArch64/, lldb/source/Plugins/Architecture/AArch64/, libc/config/linux/aarch64/, and flang/test/Driver/emit-asm-aarch64.f90.