Tznius is not more detailed than it used to be. I have been out of school for many years, but my impression is that today there is a strong emphasis on teaching about the concept of tznius and the “inside” aspect.
The reason for the seeming focus on details is twofold, in my opinion:
1. There are halachos that are not being followed. People should look at the halacha to cover your knees the same way that they look at the halacha not to turn on a light on Shabbos. It should not be considered normal for someone to consider herself Frum and to keep all other halachos but be walking around in skirts that are clearly above her knees.
You can’t expect everyone to be on the level of being able to have the highest level of “inside tznius”. That’s a level to strive for.It’s like expecting everyone to have complete kavana when they daven. But you can and should expect everyone to follow the basic clear-cut halachos of tznius.
Just like by davening, the ikar is really the kavana, but we teach everyone to at least say the words, hoping the kavana will come. And by fast days, where the real point is the teshuva and it is brought down that one is missing the point if he fasts but doesn’t do teshuva, and yet, we still tell everyone to at least fast and we hope the teshuva will come. But we don’t tell people to stop fasting until they have the proper mindset.
Perhaps it’s true that with tznius the main thing is the internal aspect, but you still have to keep the external laws. Not only that, but you can’t get to the internal concept of tznius until you are dressed tzniusly. You can’t be tznius on the inside if you are not tznius on the outside. On the other hand, being tznius on the outside can bring a person to be tznius on the inside. In fact, I disagree that tznius has become a totally external thing. It is impossible for a person to be tznius on the outside without its affecting her internal sense of tznius.
I have heard people say that others relate to them differently and that they feel differently about themselves when they are dressed tzniusly. If I’m not mistaken, I think you wrote something to that effect, LB.