Not content to rest on their laurels, the executive editors at GMD have revised and updated their policies, the new version of which can be found here. Of particular note, it is now a requirement that code must always be made available to the editor at minimum, and each manuscript must include a paragraph outlining code availability. (In practice we expect that most model code will be publicly available.) This represents a logical and incremental change to the previous policy, and once again sets a new standard for open and accessible science.
It is increasingly common for journals to have code and/or data availability policies, whether or not they are enforced. This BAMS paper is also relevant, focussing on the analysis code rather than numerical model itself.
(Disclaimer: I helped to found GMD and was an executive editor for several years. However I no longer hold this position though I continue to act as a topical editor [which basically means I handle manuscripts but don't set the editorial policy].)