Modifing RSS feed to filter one category

I’m actually looking for the same fix, any help would be appreciated. Been searching forever and nothing.

you shouldn’t modify feed-rss2.php.
but if you really want to then add the following before the loop (add a row after row 33)

<?php query_posts('cat=-55'); ?>

but better up (to still be there after updates of wordpress)
add this to your functions.php inside your theme.

function fix_feed_link($args) {
	return $args.'?cat=-55'; // add any valid feed args
}
add_filter('post_comments_feed_link', 'fix_feed_link',10,1);

this filters every call to the main feed or comment feed to not include category 55.

works for me!
some documentation on valid arguments to pass can be found here:
https://codex.www.ads-software.com/WordPress_Feeds