I think PLT is generally broad enough to include anything related to making (vs using) programming languages — the more practical topics like compiler/interpret

I think PLT is generally broad enough to include anything related to making (vs using) programming languages — the more practical topics like compiler/interpreter construction, VM implementation etc seem to obviously qualify despite not necessarily being very formal. Concretely, you can definitely design and implement your own dynamically-typed scripting language without knowing anything about type systems or formal semantics.