Limit the search pool to Uncategorized Posts in SearchWP

searchwp-customizations.php

<?php

// Limit the search pool to Uncategorized Posts in SearchWP.
add_filter(
  'searchwp\post__in',
  function( $ids, $args ) {
    $args['fields']    = 'ids';
    $args['nopaging']  = true;
    $args['post_type'] = 'post';
    $args['tax_query'] = [ [
      'taxonomy' => 'category',
      'field'    => 'name',
      'terms'    => 'Uncategorized',
    ] ];
    
    return array_merge( $ids, get_posts( $args ) );
  },
  20, 2
);