If your custom domain can experience disruption when you move to Salesforce Edge Network, you see a warning on the My Domain Setup page.
Where: This change applies to Lightning Experience and Salesforce Classic in Enterprise, Performance, and Unlimited editions.
Why: A custom domain serves your site content on a domain that you own, such as https://www.example.com. When a third-party service or content delivery network (CDN) hosts a custom domain, the target host name for that domain changes when your org moves to Salesforce Edge Network. Until your third-party service or CDN updates that value, your custom domain stops working.
To reduce the risk of outages during the move to Salesforce Edge Network, we added a warning on the My Domain page when you edit the Routing and Policies section. If your org isn’t on Salesforce Edge Network and a third party serves at least one of the custom domains in the org, you see this warning.
How: If you see this warning, coordinate with your third-party service or CDN to update the target host name for your custom domain as part of your move to Salesforce Edge Network.