Question: When can I start to use FSE? When will FSE become part of WordPress core?

Answer: Right now; if you’re happy to accept the consequences. If you want to wait until it’s in WordPress core then you’ll have to wait a little longer.

• The Site Editor beta is being developed as part of the Gutenberg plugin.
• You can start to develop your own FSE themes right now.
• But you have to be prepared for changes, some of which may be breaking.
• Some experimental FSE themes are already available.
  • from wordpress.org
  • from GitHub repositories
  • it’s pretty easy to create your own
• The Site Editor is expected to land in WordPress in version 5.9
• But parts of the Full Site Editing experience will be included in WordPress 5.8

Note: When the logic is merged into core the Server Side Rendering will be performed by different function names. This will have to be taken into account in any override logic you implement.