Having `lambda` makes the language turing-complete (in spirit)

I can implement the Y-Combinator:

(define y (lambda (f) ((lambda (x) (f x x)) (lambda (x) (f x x)))))

(y (lambda (x y) (x y)))

$ shadowenv exec env
restriction error: max call stack exceeded

Traceback:

  In main, function __shadowenv__foo.lisp
  In main, lambda
  In main, lambda
  In main, lambda
  In main, lambda
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error while evaluating shadowlisp

Why does the language need lambda at all? Having a non-recursive let already allows for non-recursive variable declarations, and I don’t think much more is required to describe every sensible configuration one would want.
The error messages are rather non-descriptive, as one can see (no lines).

I’d go even further and argue that everything in “Functions and Macros” besides let should be removed.

Full lambda & application semantics makes implementation of the format non-trivial (goes from a data specification format to a full turing-complete language, which has to be tamed by having an arbitrary maximum stack).

Start by reading the “Functions and Macros” documentation and reproducing the provided Y-combinator command to inspect the current behavior. No source files or tests are named; completion would require a decided scope for lambda support and/or descriptive error messages, followed by verification of the resulting semantics and diagnostics.