Starting in Drupal 8, we've added the notion of Experimental Modules, to help provide an early look at core features which are not yet complete. A major focus of Drupal 8.4.0 has been stabilizing these experimental modules, so that they can "graduate" to stable modules which can be installed in production and leveraged by other core and contrib modules.

Here's a document that outlays the current status of each experimental module, as well as their goals with respect to the forthcoming 8.4.0 alpha deadline (which is this coming Monday, July 31). If you're looking for a productive way to help your favourite initiative during 8.4.0's alpha/beta/RC phase, check it out!

Here's the TL;DR:

• Content Moderation: Move from alpha to beta
• Workflow: Move from alpha to beta
• DateTime Range: Move to stable
• Inline Form Errors: Move to stable
• Layout Discovery: Move to stable
• Media Entity: Move to stable (so contrib can rely on it), but hide module from UI (so end users don't accidentally turn this on solo, as it causes UX regressions)
• Migrate / Migrate UI: Get as close to stable as possible.
• Place Block: Hide module from UI (so end users don't turn it on), propose instead as patch to Block module for 8.5.0
• Settings Tray: Move from alpha to beta