Hi OP,
We promise there's no squeamishness on our part here, but we do accept that lots of folk on the boards get Brexit fatigue from time to time and so we tend to be slightly led by the feelings of the users on this one, rather than having a hard and fast 'rule' on it.
With the thread you're referring to, we did think it was more of a Brexit thread than a true AIBU, so when we got reports on it, that fact, added to the fact that it did seem to be annoying folk in AIBU helped us make that decision. We can see that it's annoying though when it's 'your' thread that gets moved.
We do try and keep a balance between 'some' and 'too many' Brexit threads on boards like AIBU and Chat. Same goes for other topics, such as trans issues and Christmas, as others here have mentioned. We know we can't please everyone all the time, so we do just do our best to keep a balance and, as we said, try to take into account 'the mood of the room'.
We also take into account what's going on in the world, with all these threads, so for example, with Christmas threads we're more likely to move them to the Christmas topic when they're posted in July and likely to cause mass blood pressure spikes! Whereas, a thread about Christmas on 15th December seems much less likely to cause mass irritation. With Brexit, there have definitely been times when the news has been quieter on the topic and there might be more general interest in a thread about Brexit, and then there are times when we have to recognise that many of our users are experiencing 'Brexit overload' and perhaps want a bit of light relief, so we might try to keep Brexit threads away from general chat areas a bit more.
There are some interesting points on this thread though, such as the comment someone made about the Brexit board being a bit too 'hidden away'. We will have a think about that. If anyone else has any other ideas on how we can improve things for everyone please do share them, we are reading.