Documentation for post meta

There is no official documented list of all possible post meta keys in core. The closest I could for anything unofficial was this answer on Stack Exchange:

You probably don’t want to use the following post meta keys:

  • _thumbnail_id – Used to store the featured image ID
  • _edit_last – Used by the Heartbeat API
  • _edit_lock – Used by the Heartbeat API
  • _wp_page_template – Stores the page template.
  • _wp_attached_file
  • _wp_attachment_metadata
  • _menu_item_{url, object, object_id, target, classes, xfn, ... }

as your custom meta keys, as they might be overwritten by the
WordPress Core.

However this is over 3 years old and not comprehensive. It’s only based on the answering user checking for all the keys in their database. You’ll notice that _wp_old_slug is missing, for example.

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