Very, very quick unscientific research suggests that there is only one query_var set for the main feed. Other feeds such as category, tag, author feeds have more than one query_var. Thus the following should kill the main feed but leave others intact.
add_filter(
'pre_get_posts',
function($qry) {
if ($qry->is_feed()) {
$fvars = array_filter($qry->query_vars);
if (1 == count($fvars)) {
wp_die( __( 'No feed available, please visit the <a href="'. esc_url( home_url( "https://wordpress.stackexchange.com/" ) ) .'">homepage</a>!' ) );
}
}
},
1
);
To remove the main comment feed, you need a small edit to check for the presence of $fvars['withcomments'].
add_filter(
'pre_get_posts',
function($qry) {
if ($qry->is_feed() ) {
$fvars = array_filter($qry->query_vars);
if (1 == count($fvars) || isset($fvars['withcomments'])) {
wp_die( __( 'No feed available, please visit the <a href="'. esc_url( home_url( "https://wordpress.stackexchange.com/" ) ) .'">homepage</a>!' ) );
}
}
},
1
);
Be warned: Barely tested. Possibly buggy. Caveat emptor. No refunds.