I thought that the `Target` is a set of compiler, processor architecture and operating system. In any case, it is convenient when all things dependent compiler collected in one place. Of course we can define another structure `Compiler`, but why? Besides, С++ depends on target OS too. DMD use one mangling in windows and other mangling in posix OS.
Thus we can say that mangling depends on target, even if we exclude from the notion "target" "compiler".