I think we should have a way to view the AFK players, like in the tab menu it might say [AFK] next to a player name. This would be useful because even though it says in chat when a person is AFK the chat goes away and this way it will be easier too see.
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Edit: SweetCrazyMom let me know that /online shows the AFK players.

We don't really want to encourage you to be on the server if you're AFK, if you know you're not going to be actively playing it would be best to just disconnect to avoid potentially annoying people who are trying to contact you and from abusing any automated things you may be stood around, etc. As a result of this it's not likely that this is going to be introduced on the server, you can feel free to do as SweetCrazyMom suggested though.