Our organisation is moving to Hyperforce does this impact our custom domain?
Yes, there is a known issue where having a custom domain using your own certificate and CDN.
To check, go to setup and search for "Domains". For the domains where the certificate is entered, if these are hosted on the Salesforce Cloud, there can be an issue moving to Hyperforce. In this instance, the migration would be blocked. Furthermore, salesforce generally will not select you for migration to Hyperforce if they believe there will be an issue. This is one of the criteria they look at.