Yes sorry, I mean customize-save-variable, custom-save-variable doesn’t exist. There’s a branch in customize-save-variable that saves all previously updated variables to the custom file if it exists.

I just took a look at cus-edit.el, there appears to be no function that can surgically serialize just one variable value to the custom file. Fixing this the right way will probably involve changing all the places that call custom-save-all and still arguably result in a breaking change.
On 4 Jun 2023 at 1:55 PM +0100, Eli Zaretskii <eliz@gnu.org>, wrote:
> > From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> > Date: Sun, 04 Jun 2023 13:36:30 +0100
> >
> >
> > As a discussion from bug #63300, it appears this long standing
> > undocumented behavior of `custom-save-variable` is coming into conflict
> > with the introduction of `connection-local-*` variables being user
> > customizable and the fact that Tramp in Emacs 29 sets them on
> > load. Here's a scenario where the combination of these behaviors results
> > in one too many surprises:
> >
> > 0. (setf custom-file "~/.emacs.d/custom.el")
> > 1. M-x load-library tramp (or install a package that transitively
> > requires tramp, without the user knowning)
> > 2. Now `connection-local-profile-alist` and
> > `connection-local-criteria-alist` are set by
> > `hack-connection-local-variables-apply`.
> > 3. M-x list-packages
> > 4. Installs a new package
> > 5. Now in addition to `package-selected-packages` being updated, 2
> > gigantic variables are also saved. Since these connection-local
> > variables are highly machine, application and connection dependent,
> > saving them into the custom file will make it very annoying to be shared
> > across multiple machines. This violates the principle of least
> > astonishment.
>
> I think the connection-local variables should be simple variables,
> initialized from corresponding user options. Then Tramp could hack
> the variables without fear of clobbering user customizations.
>
> Michael, can this be done on emacs-29 safely enough?
>
> > Expectation:
> >
> > `custom-save-variable` should only save the value of one variable
> > regardless of whether a custom file exists.
>
> How is custom-save-variable involved in the above scenario?
>
> And what is custom-save-variable? did you mean customize-save-variable
> instead? That one does save just one variable, the one you type at
> the prompt.