invisnet

The REST API is essential for any modern web framework, but with it comes a huge attack surface. WP REST API Security reduces the attack surface by disabling all the REST API endpoints by default, allowing you to enable only those actually needed. Those that are enabled require authentication by default, allowing you to choose which to make public.
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N.B. If you are using the new Block Editor you must keep nearly all the endpoints enabled for it to work, but none need be public.
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WP REST API Security

Provides a UI to control which REST API endpoints are enabled and which require authentication.

jakubkanna

This plugin forces all non-admin, non-AJAX, and non-REST API requests to return a 404 status code. 
Ideal for headless WordPress environments or sites that should expose only the API.

REST API Only

Force all non-admin, non-AJAX, and non-REST API requests to return a 404 for headless or API-only sites.

WP REST API Security vs REST API Only