Earlier quoted context omitted.
That's no solution. If you can't trust and/or verify dependencies, and they are malicious, then you have bigger problems than what a sandbox will protect against. Even if it's sandboxed and your host machine is safe, you're presumably still going to use that malicious code in production.
I'm supportive of going further - like restricting what a library is able to do. e.g. if you are using some library to compute a hash, it should not make network calls. Without sub-processes, it would require OS support.
You make the type system statically encode categories of side-effects, so you can tell from the type of a function whether it is pure computation, or if not what other things it might do. Exactly what categories of side-effect are visible this way depends on the type system; some are more expressive than others.
But it means when you use a hash function you can know that it's, eg, only reading memory you gave it access to and doing some pure computation on it.