Well, systems programming and apps programming are the domains in question, SP is about implementing the platform and infrastructure. There is no single defining characteristic since languages are used cross purposes, it's a term meant to characterise existing engineering practice. But if you really wanted an example of the least likely SP language, I'd say MATLAB or Prolog could be nominated.
Lisp and Java have been used in SP roles even though people often think of them as apps languages.
Probably most useful is to observe what usage languages see. Eg Go is used for many databases, K8s, gVisor, Docker etc.
> What is the defining characteristic of a 'systems language'
The precise technical definition is "whatever the speaker means by it". In other words, it's not a useful term.
People will come up with definitions like "whatever you can write an OS kernel/a compiler/a database management system in". So languages like Haskell, OCaml, and Java are certainly included. I don't see why Go shouldn't be.
