Thanks.

A. The same applies to the two days of Rosh Hashanah?
B. To what extent is this practical Jewish law (not from the standpoint of the current halakhic conception of Judaism)? That is: is this, say, on the level of legumes on Passover, which one can personally cancel? (Obviously these are two completely different things and the reason for permitting would also be different.)

A. There is no difference at all.
B. No. This is an enactment established by the Great Court, and an individual cannot cancel it even if its reason has lapsed. Legumes are not an enactment but a concern that was practiced at some point and was not established by an authorized court. See the Tur on legumes.

It is not clear that this is an enactment established by the Great Court; simply speaking, it is the ordinary rule that in a Torah-level doubt one rules stringently, which applied at a time when there really was doubt (when they sanctified based on sighting).
Even if we say that this was an enactment by the Great Court, the Talmud’s basic assumption was that when there is no doubt, the second festival day can be abolished, except that “they sent from there: be careful to preserve your ancestors’ custom, lest the matter again fall into disarray.” According to my historical calculation, that “they sent from there” is not the Great Court (if it were the Great Court, they would have sanctified based on sighting, and there would have been real doubt).
Rashi explained “lest the matter again fall into disarray” to mean that the secret of intercalation would be forgotten (and they would go back to sanctifying based on sighting).
Nowadays, even if the matter were to fall into disarray again, the Jews of the Diaspora would know very quickly when they sanctified the month, and they would not need emissaries to travel for several weeks to announce when the month had been sanctified,
and therefore today there is no reason for a second festival day (since there is no concern that the matter may again fall into disarray), whereas there is a reason for two festival days of Rosh Hashanah, for exactly the same reason given in the Talmud—that the matter may again fall into disarray, and they will not be able to send WhatsApp on Rosh Hashanah to announce that the month was sanctified.

It is explicit in the Talmud that this is an enactment and not merely a practice because of doubt. As for Rosh Hashanah, there is a view among the medieval authorities that already today there is no need for two days.

A. Where is there such a Talmudic passage? After all, this is the topic of setting aside eruv tavshilin and eruv techumin because of doubt, where they discuss that one way or the other, one of them is an ordinary weekday.
B. Even if you find such a Talmudic passage, I already addressed this and wrote that it is explained in the Talmud that in practice the second festival day should have been abolished were it not for “be careful to preserve your ancestors’ custom,” and that statement certainly was not said by the Great Court.
C. As for Rosh Hashanah, indeed there are medieval authorities who are lenient about this in the Land of Israel, though their reasoning has not become clear to me; outside the Land of Israel there is no one who is lenient about this.
And so we return to the law: there is a reason for Rosh Hashanah nowadays, at least outside the Land of Israel, but not for the other second festival days.

As for Rosh Hashanah, they discussed that it is one holiness, and that proves explicitly that it is an enactment. And regarding the other festivals, the matter is explicit in the passages in Beitzah (that is the whole flow of the passage. There too they disputed whether this is one holiness or two, and it is not just an ordinary discussion of the laws of doubt). When I speak of an enactment, that is not necessarily in the Sanhedrin, but in the Talmud, which has the status of the Great Court. “Be careful to preserve your ancestors’ custom” is at most the reason for the enactment (like “the Temple will soon be rebuilt,” which is also the reason for an enactment, and therefore even if it no longer applies, that does not mean the enactment can be abolished).
And see the explicit words of Maimonides, Laws of Sanctification of the New Moon 5:5, and Laws of Festival 1:21.
True, I wrote in the trilogy that there is room to abolish enactments that, because their reason has lapsed, have become a joke and are no longer relevant at all.
And regarding notification on Rosh Hashanah, there is no problem sending WhatsApp on a festival day if they lit beacon fires and sent emissaries on the Sabbath. So there is no point to these enactments at all nowadays, nor is the situation ever going to revert to what it once was.

In the book Tanya (Iggeret HaKodesh, section 26) it is explained that the second festival day is not merely a remembrance of the doubt that existed at the time of sanctifying the month, but carries a special divine light that is not revealed on the first day:
“And behold, the second festival day of the Diaspora has a superior advantage over the first festival day, in that it is an additional light from a higher level” — meaning, this is an additional divine revelation and not merely a technical enactment.
Likewise, in Likkutei Torah (Emor, s.v. ‘And you shall bring a fire-offering’) it explains that the second festival day is connected to a special spiritual illumination that comes as a result of the delay of exile:
“The second festival day of the Diaspora is a light that is delayed from descending on the first day and sparkles on the second day” — which makes clear the necessity of this day even when, ostensibly, there is no technical need for it.

Not only is the second festival day relevant, but it expresses a deeper connection to holiness דווקא in a generation of hiddenness of the divine face. In Hasidic works it is explained that exile itself created vessels for receiving a loftier light on these days, and therefore abolishing it on merely “rational” grounds reveals a lack of understanding of the spiritual character of the custom.
Ignoring the kabbalistic foundations of the second festival day and turning it into something “irrelevant” reflects a shallow approach to Jewish heritage. Beyond the straightforward Jewish law, this day serves as a vessel for revealing a higher divine light, as explained in Kabbalah and Hasidic teaching. Only someone blind to the spiritual layer of the commandments can claim that it is unnecessary.

Of course. In the book Collected Sayings I elaborated on this at length—that the second festival day also serves to prevent obesity, beautify the body, and increase wealth, as well as to bring peace among the swans in the Hula Lake. Only someone blind to these theses could think that it serves merely to add divine light.