Export One Post

Works on pages and custom post types!

In the Block Editor, the option can be found under the kebab menu:

In the Classic Editor, the option can be found in the Publish box:

And in the row actions for quick access:

Confirmed working as of WordPress 5.4.1, single and multisite.

WP-CLI

There is a core command, this plugin is not required.

wp export --post__in=123