Is it a crime for a woman to dress immodestly?

Even if she dresses like this in front of men?
Apparently the prohibition is in front of the skin. It is true that the norm has changed and perhaps there is no in front of the skin regarding what is customary and does not attract special attention. But if so, what is the source of the obligation to dress according to the rules of halakha? Perhaps the rules of in front of the skin should be viewed according to what halakha sees as a certain obstacle (and perhaps this is circular, since halakha requires in a religious society to disclose that according to halakha there is always a degree of special attraction)

Yitzhak, a woman who got sick while naked is not talking about men, of course. I don’t think there will be anyone among the Torah scholars who would explain that it is permissible to dress immodestly in front of men. Although there may be a dispute about what is considered immodest these days.

I was not asked about the situation with men. With men, it certainly depends on what arouses their thoughts, and not on any fixed halachic boundaries.
Beyond that, as Nadav Schnerb wrote in his article in Akademo, modesty prohibitions are simply not a matter of law with a blind man, but rather personal prohibitions. With a blind man, it depends on the question of whether it is reasonable to act in this way. If so, then the man should be careful. If not, then the duty of caution is upon her.

So what is the source of the halachic gedir if not a pre-skin (a section from some mitzvot thread)?

Good question. See for example here:
https://www.yeshiva.org.il/midrash/1830

What is the ruling on walking immodestly on the street (in the same manner described in the question, sleeveless, etc.)?

I wrote my opinion. In terms of her own modesty, it is difficult to draw lines. In terms of Before the Blind, if it provokes reflections in a reasonable person, there is room for discussion. Still, if it is an accepted norm, then it is difficult to prohibit. Both because somehow there are women like this on the street (and whoever is not careful somehow would have failed), and also because in accepted walking, the person in whom reflections arise is the one who needs to be careful himself (and not that the woman should be careful, because she walks normally as accepted).
There is of course the matter of the custom of the daughters of Israel (part of the Jewish religion), which is also important to observe.

I didn't understand the source of the restrictions that the Jews established as requiring coverage. From an explanation? From a rabbi?

Modest clothing was until a hundred years ago a sign of a decent woman. A decent woman dressed modestly. An immodest woman (a prostitute) dressed immodestly. It was simple and clear to everyone, Gentiles and Jews. Those who had no status, for example slaves and maidservants, were displayed naked in a slave market (search for images of a slave market on Google), perhaps so that the masters could see the goods. A decent woman, on the other hand, was dressed modestly and was never exposed. Even in bed, the halakhic guideline is to have sex under a tallit so as not to expose the woman. Modesty was part of a way of life that preceded the Torah and not of the halakhic law. Sometimes people were so poor that a woman would cut her hair naked but still when she went to market she would wrap herself in a tallit (as in the story of Rabbi Yehuda and his wife who would go to market in turns because they had only one tallit).

It is common sense that clothing reduces intra-sexual competition. When the body is not exposed, women are less threatened than other women. Where I lived, a young Balanit from the community was killed. As a result, many of the older women in the community began going to a mikveh outside the neighborhood (I heard that one of them was even involved in a High Court case against the obligation to immerse in front of a Balanit in a mikveh). It seems that some of them felt threatened by the fact that that woman would see them without clothes, with all the scars and the belly from their pregnancies. Perhaps in their subconscious they feared that that Balanit would cast an eye on their husbands and since she was able to compare herself to them, she would have an advantage in the competition for the man (for those who want a research proposal in psychology, I think there is a fertile ground here). In a promiscuous society where there is a lot of competition for the sexes on the other side, as in today's young secular society, women outnumber men and men outnumber women, and there is pressure to expose more in order to present the goods. As in the economy, when there is competition, the price goes down. In American colleges, which are the most permissive places in the world, the willingness to engage in explicit sexual acts is increasing as a result of the great competition for the attention of the opposite sex. At my current workplace, many Haredi women work, and since they dress modestly, secular women also tend to dress more modestly. It seems that they feel less threatened by the intra-sexual competition with other women.

When Derech Eretz went crazy and all women became immodest, halakhah was sucked into the resulting void. For the most part, halakhah was not required for this. The halakhic concern was more about the laws of modesty in reciting the Shema and the like, which are quite technical. The public was modest and modesty laws were not required. However, in our crazy world, rabbis are required to define definitions that most would prefer not to reach. In religious ulfans, it is known that it is always better to have a woman as the director and that she will demand the requirements. When rabbis are required to do this, voices begin to be heard that the rabbis are not modest. As a result, laws of modesty were created - the length of the skirt and sleeves, exposing or covering the calf, and more. Before that, there were no laws of modesty because there was no need for laws of modesty. This belonged to the way of life that preceded the Torah.