I'm proposing to move the feeds to controllers using these routes:

/feed/forums

/feed/topics

/feed/topic/{id}

/feed/forum/{id}

...

If the corresponding feed is disabled we will return an error 501 (Not Implemented)
And if the topic or the forum doesn't exists we will return an error 404 (Not Found)

The controllers will be put in phpbb/feed/controller/feed.php

A feed.php file will be kept to send 301 redirection to the right route for BC purpose.