bug#37656: 27.0.50; Arbitrary code execution with special `mode:'

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Wed, 16 Oct 2019 01:17:51 +0200
> Cc: 37656@debbugs.gnu.org, Emacs developers <emacs-devel@gnu.org>
> 
> The "multiple mode specification feature" dates back to:
> 9fa7bfe524 1993-09-11 Richard M. Stallman
>     (hack-local-variables-prop-line): Ignore any specification
>     for `mode:', since set-auto-mode has already handled it.
>     (set-auto-mode): Clean up.  Handle more than one `mode:' spec in -*-.
> 
> The code that my proposed patch changes has stayed untouched since
> this 1993 commit.  If we agree that disabling this feature is the
> solution here, a backported security fix should therefore hopefully be
> a one liner all the way back to version 22.1.

This feature was described as "deprecated", but where and why did we
deprecate it?