Using a terminal, you can do the following sequence of commands:
$ git clone https://github.com/laerne/desaturate_all.git
$ mv desaturate_all "~/.local/share/gnome-shell/extensions/desaturate_all@nicolas.brack.mail.be"
Then press Alt+F2, type r in the dialog window, then press Enter.
The default keyboard shortcut to toggle desaturation is <Super>+e. You can
change this shortcut key using gsettings. For example, to change the keyboard
shortcut to <Ctrl>+e, run the following command:
gsettings --schemadir \
~/.local/share/gnome-shell/extensions/desaturate_all@nicolas.brack.mail.be/schemas \
set org.gnome.shell.extensions.desaturate-all.keybindings toggle "['<Ctrl>E']"
You can now enable the extension by running gnome-shell-extension-prefs, or by
browsing to https://extensions.gnome.org/local.