Thanks. I’d be glad for a somewhat broader explanation of what a foolish custom is. Is it something unreasonable, like legumes nowadays? Fine. But a slightly different question is about rabbinic enactments: if they say something that seems foolish nowadays, do we still act that way?

You want a definition of foolishness? “A foolish custom” is not something I invented. It’s a common halakhic term. There is a custom with no rationale. A custom that has a rationale but it’s idiotic (or contradicts Jewish law—though it’s rare for a custom to uproot law). And there is a harmful custom.
A rabbinic enactment is, in principle, always binding. That’s Jewish law, not custom. But there can be changes in circumstances that justify repeal. See my article on repealing enactments.

If the real historical reason was that the Ashkenazim were concerned about an expansive view of Rabbi Yohanan ben Nuri’s opinion, and they prohibited legumes because they thought they were actually leaven, and not because of a concern about mixture—would that make a practical difference, in your view, regarding the binding force of the custom?

If we do not rule in accordance with him, there is no reason to worry about that, and certainly not to invent decrees and concerns. Besides, Rabbi Yohanan ben Nuri did not speak about legumes at all, but about rice. The straightforward explanation is different, and that is also how people explain it today.

Let me sharpen my question. There were halakhic decisors who ruled that the custom of legumes is Jewish law, so why am I not obligated to obey them, while I am obligated to obey the rulings of the Sages of the Talmud? When is it considered the Sages of the Talmud and when is it not? If it’s those who wrote or sealed the Talmud, why do I listen to them no matter what, but to somewhat later decisors I don’t have to listen? I understand what a foolish custom is, but once it was ruled as Jewish law, what authority do we have to rule otherwise apart from reason?

Binding Jewish law is either what is written in the Torah or what was established by the Great Court, by force of “do not deviate.” The Great Court has two authorities: to legislate (rabbinic laws) and to interpret (Torah-level laws). Any ruling by anyone else is not agreed upon and is not binding. Of course, you may rely on some halakhic decisor, but you are not obligated to do so. In my view, it is preferable not to do so, but rather to become competent enough to rule for yourself.
The Talmud has a special status by virtue of acceptance by the entire public. All the sages of Israel agreed not to dispute it. Therefore, de facto, it has the same validity as decisions of the Great Court. Anything after the Talmud is not binding. See the Rosh on Sanhedrin, chapter 4, section 6, at length, and a summary is brought in Choshen Mishpat, section 25.
The fact that a halakhic decisor writes something proves nothing. I will examine what he writes and see whether I agree or not. Halakhic decisors tend to give status to traditions, and in my view that tendency is problematic and excessive. Fear of Reform only intensified this, and thus the monster of legumes was born.
There is no such thing as ruling the law of legumes as binding Jewish law, because it has no source in any of those things. The decisors invented a fictitious authority for this bizarre rule, apparently out of fear of fence-breakers. But what can you do—that simply isn’t the truth.

There is no such thing as “a Jewish custom has the force of law.” That’s a distorted quotation. A custom is precisely something that is not law, so it’s a contradiction in terms. Laws are the first four parts of the Oral Torah (according to Maimonides’ division in the introduction to his commentary on the Mishnah). Customs are the fifth part. The saying is “the custom of Israel is Torah,” and the meaning is that custom is Jewish law and therefore one is obligated (or commanded) to observe it. More precisely: according to Jewish law, one must observe customs.

Rabbi, what do you think of the thesis that the prohibition of legumes on Passover—which today seems to have no apparent reason at all—is probably a prominent example of understanding deeper dimensions in an enactment or custom, beyond the visible reasons for which it was instituted or adopted? The divine providence that caused this custom to take root, to become canonical, thereby hints to us that there are substantive reasons for prohibiting legumes on Passover, beyond the concerns about leaven that existed for users of legumes in earlier times.

My view on that is written in Column 2. I explained there that legumes are not a custom but a concern.
And even regarding customs or rabbinic enactments, in my opinion the thesis of hidden reasons is an invention whose purpose is to explain why change is forbidden. Instead, one can simply tell the truth: that it is formally binding in Jewish law.
That is also why they invented divine inspiration for the Amoraim, because they feared that without it people would not accept their authority. There too, the truth is preferable: they are human beings like me and you, and their authority is based on the fact that we accepted the Talmud upon ourselves.