With the Spring '21 release, this feature is planned to become generally available.

At present, however, there is a bug in Spring '21 that wasn't noticed earlier that prevents sandbox activations and domain operations from working properly when this pilot is enabled in a production org that has more than 15 sandboxes. This is being fixed, and the plan is to fix this issue this week.

If your production org in this pilot has more than 15 sandboxes and you're creating or refreshing a sandbox within the next several days, Salesforce Support may need to turn off the pilot in your production org until after the bug is fixed. Similarly, if you're adding, editing, or removing a custom domain in a production org with more than 15 sandboxes, the pilot needs to be turned off within your production org to have those domain management operations proceed.

After the bug is fixed, Salesforce plans to enable this feature generally to all orgs that are not in the Salesforce Government Cloud, Salesforce Public Cloud, or Hyperforce.