exclude-category-rss.php

/**
 * Exclude a specific category from your RSS feed.
 * 
 * @param WP_Query $query
 *
 * @return WP_Query
 */
function exclude_category_from_rss( $query ) {
    $cat_id = 0; // Change this to be the ID of the category you want to exclude, but put a - in front. Example: -99
    if ( $query->is_feed ) {
        $query->set( 'cat', $cat_id );
    }
    
    return $query;
}
add_filter( 'pre_get_posts', 'exclude_category_from_rss' );