The review of SE-0198 has ended, and the Core Team has accepted the proposal with minor revision:
The new API for customizing playground display will be adopted, and the previous API deprecated, as described in the proposal. However, the new protocol will remain part of the standard library rather than moving to the playground support library, in order to facilitate frameworks that want to ship with built-in custom playground display capability. @cwakamo will update the proposal accordingly.
Thank you to everyone who participated in the review!