Can I get banned for only AFKing?

HugoSMP and DonutSMP both allow AFK, and both pay you for it. Both are also known to have banned accounts that never did anything else, and possibly other servers do too. It is not a certainty, but it is possible, and it is worth planning around.

Which servers this applies to

HugoSMP and DonutSMP are the two flagged in the dashboard, because those are the ones this has been seen on. They are not necessarily the only ones, and possibly other servers do the same without it ever reaching us.

Nothing on this page is specific to those two. Any server that allows AFK can still take a dim view of an account that has never done anything else, so read this as advice for whichever server you are on.

How likely is it?

No server publishes a rule saying "AFK only and you are banned", so nobody can give you a number. What there is instead is a pattern: accounts that have done nothing but AFK for a long stretch sometimes get actioned, and accounts that mix AFK with real play generally do not.

That makes this a possibility to reduce rather than a threshold you can sit just under. The line is a moderator's judgement about an account, not a timer.

How to make it less likely

1. Play on your accounts. Log in yourself, build, trade, run the odd farm by hand. It does not take much, it just has to be there.
2. Spread it around. If you run several accounts, an account that has genuinely never been played is the one that stands out.
3. Read the server's own rules. Every server publishes them, and every server changes them without telling us.

Anti-AFK movement is not the answer here

Anti-AFK exists to stop the server disconnecting an idle session, and it is very good at that. It is not a disguise. A moderator looking at an account sees the same thing either way: an account that has been online for three weeks and has never opened a chest.

Use anti-AFK to protect your uptime. Use actual play to protect the account.

What we can and can't tell you

MinecraftAFK connects as a normal Minecraft client and only does things a real player could do. See Is AFK Console Client safe? for what that covers. What it cannot do is decide how a server's staff feel about an account, and that is what this comes down to.