I don't believe there is a code of conduct for the network of federated servers, each instance has their own set of rules and can block other instances from showing up on their own instance. Some of the Mastodon instance owners are going to block Gabs domain from coming up in their instance for example.
However the list of federated servers (https://the-federation.info/info) can have their own code of conduct and prevent you from listing your instance on their website. TheFederation.info has a code of conduct that prevents a lot of content "Harmful content can be, but not limited to, malware, graphical material of minorities, abusive images, hateful content, racist content and climate denialism. The admins of this site reserve the right to decide case by case on blocking of nodes."
Although there is nothing preventing you from hosting your own list without this code of conduct. Here are some Federation server list alternatives , https://fediverse.party/en/fediverse/
(post is archived)