If you want to enable excerpt for specific page, then you can add some conditions to narrow down.
For check “front page id” you can use get_option('page_on_front');
in admin area. (is_front_page()
should not work here)
global $pagenow;
// check current screen is edit page
if ( $pagenow == 'post.php' ) {
$fron_page_id = get_option('page_on_front');
$current_page = $_GET['post'];
// check current page is match with front page
if($fron_page_id == $current_page){
add_post_type_support('page','excerpt');
}
}