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?
>