When I go to "Report player" while in STW there is an option that says "AFK". When I try this Battle Royale that option is unavailable. Is AFKing bannable in Battle ROyale?
Ban reasons always depend on what the developer deem ban-worthy. Most times something is ban-worthy if it disrupts other players in their gaming experience.
In the case of Fortnite, you don't influence other players gaming experience if you go AFK. Thereofre that is the most likely reason it is not reportable/bannable.
If you do not use any inputs for about 2-3 mins, the game will auto matically take you back to the lobby. This ensures that AFK players cannot cause any problems and cannot disrupt anyone else's gaming experience for long, meaning it is probably not bannable because you can't actually do it for long in the first place.