On obsoleting defcustoms

[Stefan Kangas]

I see two issues with how defcustoms are currently being obsoleted.
See `M-x customize-group RET dired RET', for example.

1. Why is Emacs offering to customize something that has no effect?

See "Dired Free Space Args", whose documentation says:

   This variable is obsolete since 27.1;
   ignored, as Emacs uses ‘file-system-info’ instead

Would it make sense to change ignored options such as this into defvars
instead?  (We could keep variables like "Dired Load Hook" as a
defcustom, since it still has an effect.)

2. Finding out that an option is obsolete

The obsolete options use a different face.  However, it's not obvious
that this is the meaning of that face.

One either has to run `what-cursor-position' to see that the face is
named `custom-variable-obsolete', or expand first the option itself, and
then its documentation, and then after having seen a couple of examples
induce that the face means "obsolete".

Could we add some other marker besides just the color to show that these
options have been declared obsolete?  For example by adding the text
"(obsolete)" in a prominent location.

Best regards,
Stefan Kangas