Status: refused by design
__proto__: is not FunctionalScript. { __proto__: v } is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
In an object literal, JavaScript reads __proto__: v and "__proto__": v not as a property but as the object's prototype, so { __proto__: [] } is an object that inherits from an array. FunctionalScript has no prototypes, and giving the key its own-property meaning would make accepted text mean something else than in JavaScript, so the two spellings are refused. The computed key, ["__proto__"]: v, is an own property in both languages, and is the spelling (objects).
The row acc.proto-key of refused constructs registers the refusal, and the __proto__ key gives its reason.
export default { __proto__: [] } instanceof Array;
The demo on this page compiles sample.mjs, the module above, with this build's fjs compile, and shows the line it prints and the token it refuses. Below that it shows what JavaScript answers for the same module: true.
For a property named __proto__, write the computed key, as alternative.f.js does:
export default { ["__proto__"]: [] } instanceof Array;
It answers false. The object owns a property named __proto__ and stays a plain object.
Today fjs compile prints __proto__ requires the computed key form at the refused token. The demo proposes what it should print instead: the reason at the same token, what JavaScript does there, and the allowed spelling, the line alternative.f.js writes, as help. The page compiles the alternative too, so the suggestion is one the compiler accepts.