> I understand that the new tools will work with old files, but will the
> new files work with the old tools ? i.e. will it be possible to install
> these packages to emacs 24.5 ?

No. A package that writes it like this will only be (properly) installable
on Emacs 25 and up.

Of course I'd never drop support for Emacs 24 on one of my packages just
for the sake of this minor feature. But once I start routinely requiring
Emacs 25.1 because of other features, I'll be glad to have this as a bonus.