Need to exclude a category of posts from a blog feed

I’ve inherited a site that I’m providing some support for and I want to exclude posts from a certain category in the blog feed. A custom plugin is being used in order to generate a default image when no featured image is set to the blog post and to set up the feed, which is generated when a short code is entered into a page.

I’ve tried editing the ‘post_category’ => ”, section, both with slugs and with the category id to no avail.

The website is https://english.shatil.org.il

What else can I do? The main code for the plugin is below.

<?php
/*
Plugin Name: Blog Posts Shortcode with Featured Image
Plugin URI:
Description: Custom Latest Blog Posts Shortcode with Featured Image Thumbnail. For use with Sterling Theme only.
Author:
Author URI:
Version:1.0
*/

function tt_bpfi_reqister_css(){
wp_register_style('bp_shortcode_f_i_css',WP_PLUGIN_URL."/blog_posts_shortcode_featured_image/blog_posts_shortcode_featured_image.css",$deps,'1.0','screen');
}
add_action('wp_enqueue_scripts','tt_bpfi_reqister_css');

/*-----------------------------------------------------*/
/*	Recent Blog Posts with featured image
/*-----------------------------------------------------*/
function truethemes_blog_posts_f_i($atts, $content=null) {
extract(shortcode_atts(array(
'title'   => '',
'count'   => '3',
'character_count'   => '175',
'post_category'   => '',
), $atts));

$title = $title;
$count = $count;
$truethemes_count = 0; $truethemes_col = 0;

global $post;
$exclude = B_getExcludedCats();

if ($post_category != ''){
//mod by denzel to use WP_Query class instead of get_posts, so that WPML works.
$myposts = new WP_Query('posts_per_page='.$count.'&offset=0&category_name='.$post_category.'');
}else{
$myposts = new WP_Query('posts_per_page='.$count.'&offset=0&category='.$exclude);
}

if ($title != '') {$output .= '[raw]<span class="section_title_two">'.$title.'</span>[/raw]';};

if ( $myposts->have_posts() ) : while ( $myposts->have_posts() ) : $myposts->the_post();

		$permalink = get_permalink($post->ID);

		//remove <!--nextpage--> and show only first page content
		$post_content = explode('<!--nextpage-->',$post->post_content);
		$post_content = (string)$post_content[0];
		$post_content = substr(strip_tags($post_content),0,$character_count);
		$post_content = rtrim($post_content); //remove space from end of string
		$post_content = str_replace("","",$post_content);

    //remove all shortcodes from post content.
		$post_content = strip_shortcodes($post_content);
		$thumb = get_post_thumbnail_id();

		//half width image details
		$image_width = 65;
		$image_height = 65;

		if(empty($thumb)){
		$image_url = WP_PLUGIN_URL."/blog_posts_shortcode_featured_image/images/blog_posts_featured_image.jpg";
		$image_src = truethemes_crop_image($thumb,$image_url,$image_width,$image_height);
		}else{
		//assign half image src, uses function from truethemes_framework/global/basic.php
		$image_src = truethemes_crop_image($thumb,$external_image_url,$image_width,$image_height);
		}		

$output .= '<div class="blog_posts_featured_holder">';
$output .= '<div class="blog_posts_featured_image">';
$post_thumb = "[raw]<a href='".$permalink."'><img src='".$image_src."' alt='".get_the_title()."' /></a>[/raw]";
$output .= $post_thumb;
$output .= '</div>';
$output .= '<div class="blog_posts_featured_content">';
$output .= '[raw]<strong><a href="'.$permalink.'">'.get_the_title().'</a></strong>[/raw]';
$output .= '[raw]<p>'.$post_content.'...</p>[/raw]';
$output .= '</div>';
$output .= '<div class="hr hr-dotted">?</div>';
$output .= '</div>';

endwhile; endif;
wp_reset_postdata();
//link stylesheet in footer only when shortcode is used.
wp_enqueue_style('bp_shortcode_f_i_css');

return $output;
}
add_shortcode('blog_posts_featured_image', 'truethemes_blog_posts_f_i');
?>

And addition to the child theme’s functions.php

<?php 

/* Insert your custom functions here */

function trim_excerpt($text) {
  return rtrim($text,'[...]');
}
add_filter('get_the_excerpt', 'trim_excerpt');

// Featured Image script from https://wpforce.com/automatically-set-the-featured-image-in-wordpress //

function wpforce_featured() {
          global $post;
          $already_has_thumb = has_post_thumbnail($post->ID);
              if (!$already_has_thumb)  {
              $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
                          if ($attached_image) {
                                foreach ($attached_image as $attachment_id => $attachment) {
                                set_post_thumbnail($post->ID, $attachment_id);
                                }
                           } else {
                                set_post_thumbnail($post->ID, '4262');
                           }
                        }
      }  //end function
add_action('the_post', 'wpforce_featured');
add_action('save_post', 'wpforce_featured');
add_action('draft_to_publish', 'wpforce_featured');
add_action('new_to_publish', 'wpforce_featured');
add_action('pending_to_publish', 'wpforce_featured');
add_action('future_to_publish', 'wpforce_featured');

?>