fosstodon.org is one of the many independent Mastodon servers you can use to participate in the fediverse.
Fosstodon is an invite only Mastodon instance that is open to those who are interested in technology; particularly free & open source software.
If you wish to join, contact us for an invite.
It never had one. It just displays the feed as a generic XML file. But the RSS button let you know the feed was available, and you could copy the URL and paste it into your reader/aggregator.