From what I see it's not just a warning:

$ emacs --batch --eval "(let ((debug-on-error t)) (setf vc-git-program \"git-is-not-installed\") (find-file-noselect \"whatever\"))"
Debugger entered--Lisp error: (file-missing "Searching for program" "No such file or directory" "git-is-not-installed")
  call-process("git-is-not-installed" nil (t nil) nil "--no-pager" "ls-files" "-c" "-z" "--" "whatever")
  process-file("git-is-not-installed" nil (t nil) nil "--no-pager" "ls-files" "-c" "-z" "--" "whatever")
  vc-git--call((t nil) "ls-files" "-c" "-z" "--" "whatever")
  vc-git--out-ok("ls-files" "-c" "-z" "--" "whatever")
  vc-git-registered("/home/paul/eldev/whatever")
  vc-git-registered("/home/paul/eldev/whatever")
  vc-call-backend(Git registered "/home/paul/eldev/whatever")
  #f(compiled-function (b) #<bytecode 0x15a622e462a366fe>)(Git)
  mapc(#f(compiled-function (b) #<bytecode 0x15a622e462a366fe>) (RCS CVS SVN SCCS SRC Bzr Git Hg))
  vc-registered("/home/paul/eldev/whatever")
  vc-backend("/home/paul/eldev/whatever")
  vc-refresh-state()
  run-hooks(find-file-hook)
  after-find-file(t t)
  find-file-noselect-1(#<buffer whatever> "~/eldev/whatever" nil nil "~/eldev/whatever" nil)
  find-file-noselect("whatever")
  (let ((debug-on-error t)) (setf vc-git-program "git-is-not-installed") (find-file-noselect "whatever"))
  command-line-1(("--eval" "(let ((debug-on-error t)) (setf vc-git-program \"gi..."))
  command-line()
  normal-top-level()

Paul

On Tue, 5 Sept 2023 at 21:44, Eli Zaretskii <eliz@gnu.org> wrote:

> From: Paul Pogonyshev <pogonyshev@gmail.com>
> Date: Tue, 5 Sep 2023 20:54:54 +0200
>
> To reproduce:
>
> 1. Change current directory to a Git checkout (e.g. `~/git/emacs' or something).
> 2. From the command line:
>
>        $ emacs --batch --eval "(progn (setf vc-git-program \"git-is-not-installed\") (find-file-noselect
> \"whatever\"))"
>        Error: (file-missing "Searching for program" "No such file or directory" "git-is-not-installed")
>
> Expected: Emacs is able to open the file, Git must not be essential.

Emacs does open the file; what you see is not an error, it's a
message.  The code which tries to invoke Git runs with-demoted-errors,
so any error is converted to a simple message.  If you try this
variant of your command:

  $ emacs --batch --eval "(progn (setf vc-git-program \"git-is-not-installed\") (message \"%s\" (find-file-noselect \"README\")))"

you will see:

  Error: (file-missing "Searching for program" "No such file or directory" "git-is-not-installed")
  README

That "README" at the end means that find-file-noselect did read the
file into its buffer, and the error message is just a message.

So if this somehow prevented you from doing something, please tell
more, or maybe the recipe needs more steps?