@scully So, first it's better you understand what a federation relay is. A federation relay is a third party controlled server that will share public content between Mastodon instances that join that relay.
So, if you join a relay, all public posts on your server done by you or your users are shared by all other servers that are on that same relay.
The same happens the other way around, all public posts from other servers federate with yours.
@scully So, should you join a relay, it depends on many factors, are you ok with your servers content reaching been shared to a number of servers that you have no idea who is behind them? Also, it depends on the relay itself, some can be localized. So, for example joining a relay in Japanese, Portuguese, Spanish or other languages may not be of any interest to you and only bring noise to your Federated Timeline.
@scully In terms of if you can. Yes, you can add any relays you like to your server, just be aware that relays can force you to upgrade your hosting plan as they use more server resources but it depends on the volume of data.
In terms of Mastodon.host, AFAIK the admins stopped the service of both the relay and instance. So, there is no https://relay.mastodon.host/ right now.
To find relays, I suggest searching on Mastodon Discourse https://discourse.joinmastodon.org/search?q=relays
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!