Tell Visitors When the Privacy Policy Changes

add_action( 'wp_footer', function () {
    $page_id = (int) get_option( 'wp_page_for_privacy_policy' );

    if ( ! $page_id || 'publish' !== get_post_status( $page_id ) || is_page( $page_id ) ) {
        return;
    }

    $version = (string) get_post_modified_time( 'U', true, $page_id );

    if ( is_user_logged_in() && get_user_meta( get_current_user_id(), 'wpide_policy_seen', true ) === $version ) {
        return;
    }
    ?>
<div id="wpide-policy-update" role="region" aria-label="Privacy policy update" hidden style="position:fixed;left:12px;right:12px;bottom:12px;z-index:99980;max-width:560px;margin:0 auto;padding:12px 16px;border-radius:8px;background:#fff;color:#1d2327;box-shadow:0 4px 20px rgba(0,0,0,.2);font-size:15px;">
    We have updated our <a href="<?php echo esc_url( get_permalink( $page_id ) ); ?>">privacy policy</a>.
    <button type="button" style="margin-left:8px;" data-version="<?php echo esc_attr( $version ); ?>">OK</button>
</div>
<script>
(function () {
    var box = document.getElementById('wpide-policy-update');
    var btn = box.querySelector('button');
    var version = btn.getAttribute('data-version');
    var seen = '';
    try { seen = localStorage.getItem('wpidePolicySeen') || ''; } catch (e) {}
    if (seen === version) { return; }
    box.hidden = false;
    btn.addEventListener('click', function () {
        box.hidden = true;
        try { localStorage.setItem('wpidePolicySeen', version); } catch (e) {}
        <?php if ( is_user_logged_in() ) : ?>
        var body = new URLSearchParams({ action: 'wpide_policy_seen', version: version, _ajax_nonce: <?php echo wp_json_encode( wp_create_nonce( 'wpide_policy_seen' ) ); ?> });
        fetch(<?php echo wp_json_encode( admin_url( 'admin-ajax.php' ) ); ?>, { method: 'POST', body: body, credentials: 'same-origin' });
        <?php endif; ?>
    });
})();
</script>
    <?php
} );

add_action( 'wp_ajax_wpide_policy_seen', function () {
    check_ajax_referer( 'wpide_policy_seen' );
    update_user_meta( get_current_user_id(), 'wpide_policy_seen', isset( $_POST['version'] ) ? sanitize_text_field( wp_unslash( $_POST['version'] ) ) : '' );
    wp_send_json_success();
} );