Based on the commit message [0], and the references to "user code" in this document, my guess is that user programs have or will have access, but it's not finalized enough to be documented.

That being said, I suppose if you're developing an internal API for a compiler/interpreter, your "users" could be other parts of the project rather than language users.

https://github.com/sbcl/sbcl/commit/7f65522a16d857e41aa61cd0...