For the meta keys mentioned (_wpas, reddit, etc), there is not a standard list, per se. Each plugin can add any meta key and, in fact, any user can via “Custom Fields” in the WP admin. Keys preceded with an underscore are “private” and are not listed in the Custom Fields area of the wp-admin (on self-hosted sites at least). They’re meant for plugins to use to store data associated with a post. Keys without an underscore prefix would be displayed in the UI natively.
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