php – Is there a way to download only Rest API?
WP REST API depends on WordPress, as it is just another interface for interacting with its other APIs. If BackPress (https://backpress.org/) was still in development, you could have had some chance with it, but it seems the project is abandoned.
The plugin that was merged with WordPress Core is available here: https://github.com/WP-API/WP-API but is also abandoned, so you will not get the latest stuff that’s in 5.8. I don’t know whether this would be of help to you, as I don’t know your main goal, but I would rather try to extract whatever I need from nowadays’ Core, than messing with the old plugin.