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I don't think they're actually allowed to ban you for a long username, as mentioned above but I think they would always give you the option to change your name for an alternative. I don't think it would be fair for you to get banned, so I don't think they'll ban you. :)
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No. You'll be fine and it was for more than one day as I registered many in 2009 myself.
None of mine have been banned such as "UnidentifiedFlyingObject" and "AllAmericanReject".
We were allowed to register them, you didn't even have to do some sort of glitch you were just able to and when it happened ludus stated "Long names are allowed" when asked about it. I know he's no longer staff but they can't just go back on their word.
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I'd highly doubt they would ban you seeing they did originally allow you to register it and termination of the account would be hugely unfair.
You should be fine, if for any reason it isn't you are most likely entitled to a name change and I'm sure they will allow you to do so. :)