Comparing this to Borland or Microsoft is bizarre: Microsoft and Borland actually do have proprietary C compilers that they do not make publicly available in source code form. Those compilers in turn have all kinds of non-standard, proprietary extensions.
The one thing that sets C apart from Rust and Go is the presence of an official ISO standard. But this isn't material to the definition of "proprietary" in the open source world. And, well, that ISO standard is itself copyrighted.
[1]: https://github.com/golang/go/blob/master/LICENSE
Edit: You shouldn't delete and re-write your comments. Here's the original comment I responded to:
> That's a dubious claim, and irrelevant besides. They are proprietary, in the standard meaning of the word, because a single private organization has exclusive control over each, but whatever word you choose to describe that is up to you.
> Google owns Go in a way that has never been true of Microsoft (or Borland or GNU or Apple...) when it comes to C.
Edit 2: For the edited comment: you're setting up a distinction that does not matter for programming languages that do not have formal specifications. There is no "interface" to distinguish against, and the claim that Rust (or Go, or Python, or any other language without an ISO spec) is tacitly reserving all rights to the design of the language itself is completely unsubstantiated.