bug#57503: 28.1.91; package-selected-packages should not be saved to custom-file

[Lars Ingebrigtsen]

Stefan Kangas <stefankangas@gmail.com> writes:

> - Keep the old defcustom, and add a new multisession variable.
>
> - If the old variable exists, use it.  Otherwise the new one.
>
> - Add a new command that "upgrades" the variable into a multisession
>   variable.  Make it say that it will break things on Emacs < 29.  Don't
>   run it automatically.
>
> - In Emacs version N (e.g., 5 major versions from now?), upgrade any
>   user automatically, without prompting, and make the defcustom
>   obsolete.

Hm...  I think this has the disadvantage that it both requires user
intervention, but in addition it'll force you into one usage pattern or
another at some point.

I think perhaps Stefan M's suggestion of adding a new user option to
control the behaviour here is the only practical way forward.