This feature is disabled, because Discord has refused to give us message content intents.

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AFK command

/afk [reason]
If someone mentions you, they will see how long you’ve been AFK for, and for what reason.

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Auto-AFK command

Now by default, if you send a message in any channel, the bot will remove your AFK status. If you want to disable this behaviour, you can use the auto-afk command. 
/autoafk
If you disable auto-afk, you can remove your AFK status using the /afk command.