bug#81632: 31.0.91; outline-view-change-hook is marked obsolete, without an adequate replacement

The hook `outline-view-change-hook' was deprecated in 29.1 in commit
53b1e6f96cb03a461061e1e21cd185dd8a82ce6b. However there does not seem to
exist an adequate replacement. Sometimes there is a need to run some
code when folding or unfolding sections of a buffer.

For instance the new `markdown-ts-mode' has this comment and code:

    ;; NOTE: `outline-view-change-hook' was obsoleted in 29.1 (commit
    ;; 53b1e6f96cb) on the grounds that only lazy-lock used it, but no
    ;; replacement was provided and outline.el itself still runs the hook
    ;; in 10+ places.
    (with-suppressed-warnings ((obsolete outline-view-change-hook))
      (add-hook 'outline-view-change-hook
                #'markdown-ts--outline-view-change nil t))

Would it make sense to revert the deprecation or offer an alternative
API to monitor folding and unfolding? Thanks!

In GNU Emacs 31.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2026-08-16
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Debian GNU/Linux 13 (trixie)

Configured using:
 'configure --prefix=$HOME/.local/share/emacs-install
 --without-compress-install --with-tree-sitter=ifavailable
 --with-native-compilation --with-dbus --without-selinux
 --without-threads --disable-gc-mark-trace --without-gsettings
 --without-gpm --with-cairo --with-cairo-xcb --with-xinput2
 --with-x-toolkit=gtk3 --without-toolkit-scroll-bars 'CFLAGS=-O3
 -mtune=native -march=native''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG LIBOTF LIBSYSTEMD
LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP
SOUND SQLITE3 TIFF TREE_SITTER WEBP X11 XCOMPOSITE XDBE XFIXES XIM
XINERAMA XINPUT2 XPM XRANDR XRENDER XSHAPE XSYNC GTK3 ZLIB