WORK IN PROGRESS: Are you annoyed by all the scripts that are loaded into your page with Contact Form 7? Here is an incredibly simple, dependency-free (no jQuery) replacement that implements CF7's REST API to provide AJAX form validation and feedback.

cf7ajax.js

/**
 * Please note: This file uses ES6 code structures and likely needs to be transpiled for better browser support in your application.
 *
 * To use this, you'll need to enqueue it with a localized variable that passes the CF7 feedback endpoint, for example,
 * 

wp_register_script( 'cf7-ajax-script', 'path/to/cf7ajax.js', [], '0.1', true );
wp_localize_script( 'cf7-ajax-script', 'cf7FeedbackEndpoint', get_rest_url( null, 'contact-form-7/v1/contact-forms/###/feedback' ) );
wp_enqueue_script( 'cf7-ajax-script' );

 * And don't forget to disable the default CF7 scripts,

define( 'WPCF7_LOAD_JS', false );

 * Have fun! I will try to finish this soon and write a Medium article that better explains its usage.
 */
( function( feedbackUrl ) {
    "use strict";

    const intercerptCF7Form = ( e ) => {

        let form = e.target || e.srcElement;
        
        if ( ! form.classList.contains( 'wpcf7-form' ) ) {
            return;
        }

        e.preventDefault();
        e.stopPropagation();

        let data = new FormData( form );
        if ( e.submitter && e.submitter.name && e.submitter.value ) {
            data.set( e.submitter.name, e.submitter.value );
        }

        let formXHR = new XMLHttpRequest();
        formXHR.myForm = form;
        formXHR.addEventListener( 'load', doFormFeedback );
        formXHR.addEventListener( 'error', doFormError );
        formXHR.responseType = 'json';
        formXHR.open( 'POST', feedbackUrl.replace( '###', data.get( '_wpcf7' ) ) );
        formXHR.send( data );

        return false;

    };

    const doFormFeedback = ( e ) => {

        let { target } = e;
        let { status, response, myForm } = target;

        console.log( e );

        if ( status !== 200 ) {
            response = { message: 'An error occurred submitting your form. Please try again.', status: 'error' };
        }
        
        if ( response.message ) {
            let output = myForm.querySelector( '.wpcf7-response-output' );
            output.innerHTML = response.message;
            output.setAttribute( 'aria-hidden', 'false' );
        }

        if ( response.status === 'mail_sent' ) {
            // do something
            myForm.classList.add( 'sent' );
            refreshValidation( myForm, [] );
            return;
        }
        if ( response.status === 'validation_failed' ) {
            refreshValidation( myForm, response.invalid_fields );
        }

    };

    const refreshValidation = ( form, invalidFields ) => {

        let fields = form.querySelectorAll( '.wpcf7-form-control-wrap' );
        for( let field of fields ) {
            field.classList.remove( 'wpcf7-not-valid' );
            let message = field.querySelector( '.wpcf7-not-valid-tip' );
            if ( message ) {
                field.removeChild( message );
            }
        }

        for( let field of invalidFields ) {
            let into = form.querySelector( field.into );
            if ( ! into ) {
                continue;
            }
            into.classList.add( 'wpcf7-not-valid' );
            
            let message = document.createElement( 'span' );
            message.classList.add( 'wpcf7-not-valid-tip' );
            message.setAttribute( 'aria-hidden', 'true' );
            message.innerHTML = field.message;
            into.appendChild( message );
        }

    };

    const doFormError = ( e ) => {
        // todo: something. i told you it was WIP
    };

    document.addEventListener( 'submit', intercerptCF7Form );

} )( cf7FeedbackEndpoint );