In Emacs 25, substitute-command-keys removes text properties of strings containing quotes:

> In Emacs 25:
(substitute-command-keys (propertize "`a'" 'prop 'val))
"‘a’"

> In Emacs 24.5:
(substitute-command-keys (propertize "`a'" 'prop 'val))
#("`a'" 0 3 (prop val))

As Eli pointed out on the mailing list,

> Anything that is substituted has its test properties removed:
> 
>   (substitute-command-keys (propertize "\\[forward-char]" 'prop 'val))
>    => "C-f"
> 
> The above is from Emacs 24.5.
> 
> I think patches are welcome to reinstate the properties.

This is an issue in particular for docstrings:

(defcustom my/pretty/set/option nil
  "Demo text properties in customize."
  :group 'emacs
  :type `(set (const :tag ,(concat (propertize "Title of option 1 (bold on both 24.5 and 25)" 'face '(:weight bold))
                                   "\n   "
                                   (propertize "Docs of option 1, in a smaller font on both 24.5 and 25." 'face '(:height 0.9))))
              (const :tag ,(concat (propertize "Title of option 2 (`bold' on 24.5 but not 25)" 'face '(:weight bold))
                                   "\n   "
                                   (propertize "Docs of option 2, in a smaller font on 24.5, but not 25." 'face '(:height 0.9))))))

This example uses text properties on `:tag's to make the customize buffer more readable. This works fine in GNU Emacs 24.5, but it stopped working on master (and emacs-25), because of substitute-command-keys replacing quotes and as a side effect dropping text properties.

Clément.