Even if she dresses that way in front of men?
Seemingly the prohibition would be “do not place a stumbling block.” True, the norm has changed, and perhaps there is no issue of “do not place a stumbling block” regarding something people are used to and that does not attract special attention. But if so, what is the source of the obligation to dress according to the rules of Jewish law? Maybe we should relate to the rules of “do not place a stumbling block” based on what Jewish law sees as a certain stumbling block (though maybe that’s circular, since in a religious society, exposing what Jewish law says should not be exposed will always carry some special degree of attraction).

Yitzhak, when it says that a woman separates her dough-offering while naked, obviously it is not talking about in front of men. I don’t think anyone who observes Torah and commandments would explain that it is permitted to dress immodestly in front of men. Though there may be disagreement about what counts as immodest nowadays.

I wasn’t asked about the situation in front of men. In front of men, it certainly depends on the question of what provokes improper thoughts in them, and not on any fixed legal definitions.
Beyond that, as Nadav Shnerb wrote in his article in Akdamot, the prohibitions of modesty are, in the simple sense, not derived from “do not place a stumbling block” but are intrinsic prohibitions. “Do not place a stumbling block” depends on whether it is reasonable to walk around that way. If it is, then the man should be careful. If not, then the duty of caution is on her.

So what is the source for the legal parameters, if not from “do not place a stumbling block” (which section of the 613 commandments)?

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

What is the law regarding walking immodestly in the street (in the same way described in the question, sleeveless, etc.)?

I wrote my opinion. In terms of her own modesty, it is hard to draw lines. In terms of “do not place a stumbling block,” if it provokes improper thoughts in the average person, there is room to discuss it. And nevertheless, if this is an accepted norm, then it is difficult to prohibit it. Both because in any case there are such women in the street anyway (and someone who is not careful would stumble regardless), and also because when it is a normal accepted way of walking around, the person in whom improper thoughts are aroused is the one who needs to be careful himself (rather than the woman needing to be careful, since she is walking around normally, as is accepted).
There is of course also the matter of the custom of the daughters of Israel (part of Dat Yehudit), which also ought to be preserved.

I didn’t understand what the source is for the parameters that the Sages established as requiring covering. By reasoning alone? Rabbinic law?

Modest dress was, until a hundred years ago, a sign of a respectable woman. A respectable woman dressed modestly. A disreputable woman (a prostitute) dressed immodestly. It was simple and obvious to everyone, Jews and non-Jews alike. Those who had no status, for example slaves and maidservants, were displayed naked in the slave market (search for pictures of a slave market on Google), perhaps so that masters could inspect the merchandise. A respectable woman, by contrast, was dressed modestly and was never exposed. Even in bed, the halakhic instruction is to have relations under a tallit so as not to expose the woman. Modesty was part of basic decency that preceded the Torah, not part of Jewish law. Sometimes people were so poor that the woman would separate the dough-offering while naked, but even then, when she went out to the market she would wrap herself in a tallit (as in the story of Rabbi Yehuda and his wife, who would take turns going out to the market because they only had one tallit).

It would seem that clothing reduces intrasexual competition. When the body is not exposed, women feel less threatened by other women. In the place where I lived, they appointed a young female mikveh attendant from the community. As a result, a large portion of the older women in the community began going to a mikveh outside the neighborhood (I heard that one was even involved in a High Court petition against the requirement to immerse in front of a mikveh attendant). It seems some of them felt threatened by the fact that this woman would see them without clothes, with all the scars and the belly after pregnancies. Maybe subconsciously they feared that this attendant might set her eyes on their husbands, and since she was able to compare herself to them, she would have an advantage in the competition over the man. (For anyone looking for a research proposal in psychology, it seems to me there is fertile ground here.) In a promiscuous society, where there is strong competition over the opposite sex, like among today’s young secular society, women compete over men and men over women, and pressure is created to expose more in order to display the merchandise. As in economics, when there is competition, the price goes down. In colleges in the United States, which are the most promiscuous places in the world, willingness for blatant sexual acts increases as a result of the intense competition for the attention of the opposite sex. At my current workplace there are many Haredi women working, and because they dress modestly, even the secular women tend to dress more modestly. It seems they feel less threatened by intrasexual competition with the other women.

When basic decency went crazy and all women became disreputable, Jewish law was pulled into the vacuum that was created. Usually Jewish law was not called on to deal with this. The halakhic interest was more in the area of the laws of nakedness regarding reciting Shema and the like, which is fairly technical. The public was modest, and there was no need for laws of modesty. But in our crazy world, rabbis are required to define definitions that they usually would prefer not to have to reach. In Religious girls’ high schools, it is well known that it is always preferable that there be a female principal and that she make the demands. When rabbis are required to do this, people start saying that the rabbis themselves are not modest. As a result, laws of modesty were created—the length of the skirt and sleeves, whether the calf may be exposed or must be covered, and so on. Before that there were no laws of modesty because there was no need for laws of modesty. It belonged to basic decency that preceded the Torah.