Hi Daniel,
Could you clarify what you mean by old design guidelines? We didn’t remove any of the old design guidelines with the redesign. We actually merged design and developer documentation and guidance into a single location. Our new design system documentation site is not targeted for anything or anyone using “Atlaskit” any longer — we’ve actually expanded our audience to designers, developers, content designers, and more. 
You can read more about this here: Our new home for Atlassian Design System - #11 by ngault
Modal dialog usage guidelines are available here: https://atlassian.design/components/modal-dialog/usage
Please let me know if this is not what you are looking for~