It's not a rule on paper that decides what's allowed, it's a human with their own history and beliefs who has that authority - if you can't board the flight it doesn't exactly help you that the airline apologizes to you afterwards because the decision to throw you off was clearly wrong.
People get thrown off flights for the most ridiculous reasons, so my personal advice is: Dress like you always do, don't bother with the small chance that you'll meet someone wanting to abuse their power, and if bad luck hits you, revel in the fact that you now have a good story to tell, while the power-tripping simpleton who threw you off the plane is probably getting fired for it.
If you want to be more specific:
- Anything with a political slogan.
- Anything with a religious slogan.
- Anything that can be misunderstood as a political of religous slogan.
- Anything that can be seen as hate speech (no Metal band t-shirts)
- No ripped T-Shirts
- Anything that associates you with a socially accepted target of hate speech.
- Anything that is somewhat sexual in nature (no breast hair).