Tried to search past proposals for this but couldn't find one The short
version: Most editors / syntax highlighting engines, non-engine parsers,
node.js - they all support a leading shebang line in .js files. With the
advent of ES6 modules it's the final holdout where node has to patch the
script source to make V8 parse the code (and thus producing a mismatch
between what's on disk and what the engine sees).

Is there a downside to allow any script or module to begin with the magical
#! bytes? It should not break existing scripts because it's invalid syntax
today & the parse overhead should be fairly limited.
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Tried to search past proposals for this but couldn't find one. The short
version: Most editors / syntax highlighting engines, non-engine parsers,
node.js - they all support a leading shebang line in .js files. With the
advent of ES6 modules it's the final holdout where node has to patch the
script source to make V8 parse the code (and thus producing a mismatch
between what's on disk and what the engine sees).

Is there a downside to allow any script or module to begin with the magical
#! bytes? It should not break existing scripts because it's invalid syntax
today & the parse overhead should be fairly limited.