Is there anything in existence which has a version of this idea? It makes a ton of sense to me, but you are right that it would be practically impossible to do in a current language.
Austral is a really cool experiment and I love how much effort was put into the spec which you've linked to. It explains the need for capabilities and linear types, and how they interact, really well.
Yes, but you can't enforce this at the language level if your objective is security (at least not for natively-compiled languages). You need OS-level support for capabilities, which some OSes do provide (SeL4, Fuchsia). But if you're in a VM rather than native code then you can enforce capabilities, which is what Wasm does with WASI.
