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Every reasonable. Maybe I am wrong, but tco usually means no more stacktraces?

Not at all! Elixir stacktraces are much nicer than JavaScript ones.

No, only dumb implementations, Scheme and other languages that require TCO have no issues mapping the debugging metadata to proper calls.

JavaScriptCore has tail calls

Nothing that one can rely on for portable code.

stackful coroutines, an "await" keyword that only awaits one layer of awaitables instead of infinitely many layers, etc.