Constant REF_NAME_OUTSIDE_REFS 

pub const REF_NAME_OUTSIDE_REFS: &str = "ref name must start with refs/ (refs outside refs/ \
     written by older servers are no longer served; see the migration notes)";

The public message for a ref name outside SERVED_REFS_PREFIX (SPEC-REFS §2), on every binding; mkit serve sends it as-is. It points an operator whose repo holds such refs, written by an older mkit serve, to the migration notes (docs/CLI.md, “Refs outside refs/”); it names no server path.