But a function in one part a file shouldn't be broken by the use of bad practices in a completely different part of the file. If I paste a function that works in the REPL into a file with other code, that function should always continue to work (unless it intentionally uses global variables, or another feature that is supposed to interact with other code).

Enforcing best practices by throwing errors where other languages wouldn't (like Go does) is one thing. Enforcing them by introducing difficult-to-track bugs into code that happens to be in the same file as other code which uses bad practices is another. This is a bug, not a feature.