Add unique stopword(s) for a single SearchWP Engine

searchwp-customizations.php

<?php

// Add unique stopword(s) for a single SearchWP Engine.
add_filter( 'searchwp\query\search_string', function( $search_string, $query ) {
	// Remove "apple" and "orange" for my_engine searches.
	if ( 'my_engine' === $query->get_engine() ) {
		$search_string = str_replace( [ 'apple', 'orange' ], '', $search_string );
	}

	return $search_string;
}, 20, 2 );