I have enabled feeds (top circle) and selected a News Feed (bottom circle):

And yet I get this when browsing to /feed.php:

Strangely enough, the feed will only work if I enable this option:

Extensions:

• Advanced BBCode Box
• Auto Database Backup
• gn36 - Customize first post edit permissions
• mChat Extension
• Pages
• Permission: Read other users' topics
• Posting Template
• Sortables Captcha

Happens on two environments - a XAMPP localhost server and a remote web server.

On a side note, since https://wiki.phpbb.com/Practical.Displaying_posts_and_topics_on_external_pages is convoluted, outdated, and meant only for phpBB experts, the community will rely more on external Feed RSS widgets and such that require these Feed part of phpBB to work well.