OK, lots of things to react to.
> Not defining `custom-file' _would_ change the
> behavior. Setting `custom-file' to your init
> file would keep the current default behavior of
> Customize using your init file.
IMHO, that's a bit more disruptive (Po Lu, you made me think about that ;-) )...
And using the NEWS to explain the change might have people getting mad.
My approach, as detailed above. might be less "aggressive", right? Wouldn't be people happier with it?
Like in "I didn't say anything about custom-file, so if I don't, nothing changes"
Then they can eventually set custom-file to "~/.emacs.d/custom.el" to migrate (NEWS might be a nice place to advertise this)
With that I'd be more than happy, because I would be able to define my OS specific custom-file and have it loaded.
> Why byte-compile your init file?
That was something I wasn't thinking about. I never byte-compiled my init file, nor would I think that custom-file should be byte-compiled either :-)
/PA