You can check the value of the `post_status` query variable and make sure that it's set to `trash`:

    function wpse239286_trash_column( $columns ) {

    	// Bail if we're not looking at trash.
    	$status = get_query_var( 'post_status' );
    	if ( 'trash' !== $status ) {
    		return $columns;
    	}
    
    	return array_merge( $columns, 
    		array( 'trash_column' => __( 'Trash Column', 'text-domain' ) )
    	);
    }
    add_filter( 'manage_posts_columns' , 'wpse239286_trash_column' );
    add_filter( 'manage_pages_columns' , 'wpse239286_trash_column' );