> Vi has % to jump to a matching parenthesis as pretty ancient feature.

I know. It helps, but it is far from being good enough. Making things like slurp is still a pain without proper editor support, and this is very common to do in Lisp.

> All the code you see here was written using little more than indentation via Lisp mode and % for paren matching:

Yeah, and K&R written Unix with only ed. My point still is valid: Nix is more readable than Lisp without proper editor support.

Also, any half decent Lisp dialect can tell you where an unbalanced parenthesis lies; you shouldn't have any difficulties hunting for this in a large file.

  $ txr unbal.tl 
  unbal.tl:8: syntax error
  unbal.tl:8: unterminated expression
  unbal.tl:8: while parsing form starting at line 5
  $ pr -Tn unbal.tl
      1   (defun foo()
      2   
      3     )
      4   
      5   (defun bar()
      6     (let (x)
      7       )

I don't think this is a great error message. I use Clojure everyday and I still had to look 2 times to find the error.

If it was something like this, this would be much better:

  Unmatched parenthesis from starting at line 5.
  Maybe you wanted to do something like this?
      1   (defun foo()
      2   
      3     )
      4   
      5   (defun bar()
      6     (let (x)
      7       )
               ^

This being a very basic code. In reality you will have multiple function calls inside more complicated functions (like a package definition in Guix), so you will end closing 10+ parenthesis and catching them can be hell.