show only one category posts in admin

to show one specific category posts in admin all posts page (edit.php?post_type=post) you need to join term_relationships table by filtering posts_join and adding where clause by filtering posts_where.

eg.

add_filter( 'posts_join', 'hide_attachments_wpquery_join' );
function hide_attachments_wpquery_join( $join, \WP_Query $query ) {
    global $wpdb;
    if( is_admin() && $query->query['post_type'] == "post" ) {
        $join .= " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id)";
    }
    return $join;
}
add_filter( 'posts_where', 'hide_attachments_wpquery_where' );
function hide_attachments_wpquery_where( $where, \WP_Query $query ) {
    global $wpdb;
    if( is_admin() && $query->query['post_type'] == "post" ) {
        // change 6 with your category/term id
        $where .= " AND ( $wpdb->term_relationships.term_taxonomy_id IN (6) )";
    }
    return $where;
}