>> Second package released a couple of days that refers to future features.
>>
>> track-changes says:
>>
>> ;; Package-Requires: ((emacs "24"))

"Package-Requires:" does not intend to mean that the package is
guaranteed to work with that release.  Instead it's intended to mean
something more like "it's guaranteed not to work with an older release".

>> Is there an ELPA infrastructure testing regime,

AFAIC, the burden of testing on older releases falls on users of those
older releases.