Answer by Oleg Butuzov for Disable only the main feed?

using rewrite rules you can disable a url pattern that match main feed.

add_filter('rewrite_rules_array', 'rewrite_rules_array_feed_disable');function rewrite_rules_array_feed_disable($rules){    unset($rules['feed/(feed|rdf|rss|rss2|atom)/?$']);    unset($rules['(feed|rdf|rss|rss2|atom)/?$']);    return $rules;}