Answer

Hello.
I understand that this is an old question, and I don’t know why it wasn’t answered. I probably missed it.
A-B. I don’t think the concept of the spirit of Jewish law has any real meaning. It is true that in anyone’s interpretation there enter general assumptions that he has accumulated from his environment and from his general halakhic experience, and that might be called the spirit of Jewish law. But there is no such universal thing. These are expressions that reflect the position of a particular halakhic decisor, and another may disagree with them.
C. So you see that you yourself understood that Maimonides’ words are not Jewish law but social policy that depends on norms. The rule is that Jewish law must be anchored in a verse, an exposition, or a rabbinic enactment/decree. If a halakhic decisor, whether among the medieval authorities or later authorities, interprets things on the basis of verses or rabbinic law, then that is Jewish law. If he writes that this or that seems right to him, then it is at most a recommendation and has no binding authority. True, sometimes even when they hang something on a verse, they do not really mean interpretation but rather expansion (the spirit of the verse). Here one needs interpretation based on reasoning, and I don’t have rules for that.

Does the concept of “the spirit of Jewish law” actually exist, and does it carry halakhic weight? That is, if according to Talmudic passages and the opinions of the medieval authorities, some leniency or ruling does in fact emerge—one that was not accepted from time immemorial, and especially if that ruling also fits with the enlightened and modern “spirit of the times”—is there some parameter of “the spirit of Jewish law” that neutralizes the halakhic analysis of the passage.
The question was prompted by Rabbi Dov Lior’s response to the new book by Rabbis Stav about relationships and family, in which they adopted major leniencies (marriage between a religious man and a secular woman, in vitro fertilization for an unmarried woman, making do with the commandment of “in the evening” with only 2 children), which indeed seem (if that is in fact the case) to stand up to the test of “dry Jewish law” (what is that?) in the give-and-take of the Talmud and the halakhic decisors, but are not in keeping with the “spirit of Jewish law” (wet Jewish law?) and, in Rabbi Lior’s words, surrender to the “spirit of the times.”
 
[Rabbi Lior’s words can be found here (not in the article itself, but in the attached file showing the letter itself) http://www.srugim.co.il/240512-%D7%94%D7%A1%D7%A4%D7%A8-%D7%A9%D7%9C-%D7%94%D7%A8%D7%91%D7%A0%D7%99%D7%9D-%D7%A1%D7%AA%D7%99%D7%95-%D7%9B%D7%A0%D7%99%D7%A2%D7%AA-%D7%94%D7%94%D7%9C%D7%9B%D7%94-%D7%9C%D7%94%D7%9C%D7%9B%D7%99
 
In summary:
A. Does the concept of “the spirit of Jewish law” exist at all? Is it a halakhic parameter?
B. How is it determined? Isn’t every halakhic innovation, at least at first, against the “spirit of Jewish law”?
C. Clearly the norms in the days of the Sages and in our day are different. How do we distinguish what in the words of the Sages is a halakhic obligation, and what is only a norm according to which the Sages operated and wrote their words, but which is not binding? (For example, no one in the world would think that Maimonides’ words, “A husband should prevent his wife from this and should not allow her to go out except once a month, or several times a month according to need,” are a rabbinic decree, and certainly not relevant today; rather, Maimonides was obviously speaking according to the norms current in his time.)