Conditionally remove an Engine Source before searching in SearchWP

searchwp-customizations.php

<?php

// Conditionally remove a SearchWP Engine Source.
add_filter( 'searchwp\query\before', function( $query ) {
  // Applies only if `myflag` GET variable exists.
  if ( ! isset( $_GET['myflag'] ) ) {
    return;
  }
  
  // Remove Posts from Engine.
  $source = \SearchWP\Utils::get_post_type_source_name( 'post' )
  $query->get_engine()->remove_source( $source );
}, 20, 2 );