Opt Out of Instanceless My Domain URLs
Where: This change applies to Lightning Experience and Salesforce Classic in Group, Professional, Enterprise, Performance, Unlimited, and Database.com editions.
When: These critical updates will be automatically activated on March 16, 2019.
How: From Setup, enter Critical Updates in the Quick Find box, and then select Critical Updates. For Remove Instance Names From URLs for Visualforce, Community Builder, Site.com Studio, and Content Files, click Deactivate.
You can also opt out of instanceless URLs for sandbox orgs. For Stabilize the Hostname for My Domain URLs in Sandboxes, click Deactivate.