This will have at least `WARNING` severity, as it violates the specification and should make the validation fail. I would say that a missing version would also definitely be an `ERROR` severity, as it will actually make the release data unusable on most application (I think libappstream will even refuse to read such a release). If you want to make both takes errors, that would be fine with me too though.

Please split this into `release-version-missing` and e.g. `release-time-missing`, in order to adjust the priorities individually and give a better explanation (especially for the "either `date` or `timestamp` case).