As a third culture Millennial, I'd like to offer a slightly different perspective:
I see a few people here commenting that people should simply 'know' what is & isn't appropriate to wear in certain situations. But exactly how do you gain this 'common sense' in the first place? In @rachylou's example, her father's notion to wear his 'Sunday best' had to come from somewhere - likely authority figures, like his pastor, parents or teachers.
The thing is, with the world becoming more multicultural & accepting of nontraditional backgrounds, we cannot assume these things anymore. For example, I repeatedly see American wedding advice that only the bride should wear white. But in the UK, it's not uncommon to see bridesmaids intentionally decked out in white! Most Americans I mention this to are already well aware of the origin of this tradition, so how could they end up with a completely opposite interpretation of the resulting dress code?
Which brings me back to Millennials - it seems we cannot trust these previously reputable sources of common sense to be up-to-date. So where should we Millennials (being blamed for the death of the dress code) go for advice? The Internet has its caveats (not YLF, ofc ).
I agree that banning individual items usually isn't as effective as general guidelines, but you can still give a range of examples that embody the spirit, right? Communication is critical - @Gaylene's experiences are not examples of insubordination but of people feeling like they're not being listened to. If they feel like they can't come to you with questions or suggestions about any of your rules, that's when you start to get dissenters.
TL;DR I see 'killing the dress code' as a response to dress codes that are unable (or unwilling) to change with the intersectionality of these times, rather than a jab at having rules at all.