Is it? On my system the only things depending on binutils (which libbfd is part of) are gcc, clang and perf (seems archlinux doesn't care).

That makes it arguably not a system library, so it's fair that Debian would want to steer clear of that interpretation.

What is Clang using it for? I thought it had its own suite of that.

This... is some weirdness in Arch's clang package.

It depends on gcc, for some reason, and that depends on binutils?

(and I have no idea if anything here uses libbfd, since it's just a part of the binutils package and not split out)

Clang uses part of the host compiler toolchain; gcc is the default compiler on Arch included in the base package.

https://stackoverflow.com/a/38291698/4179075

Maybe it depends on gcc so it can use gcc's assebler for inline assembly?

no its for linking. basically clang unless configured in the source it does not know what link options to give. Actually even rust uses gcc or clang to do same thing.

Clang will also use libgcc by default under Linux to allow e.g. stack unwinding to work in mixed Clang + GCC applications.