Lo Bashomayim He

Sukka 44a-b:

אתמר ר' יוחנן ור' יהושע בן לוי חד אמר ערבה יסוד נביאים וחד אמר ערבה מנהג נביאים תסתיים דר' יוחנן הוא דאמר יסוד נביאים דא"ר אבהו א"ר יוחנן ערבה יסוד נביאים הוא תסתיים א"ל ר' זירא לר' אבהו מי א"ר יוחנן הכי והא"ר יוחנן משום ר' נחוניא איש בקעת בית חורתן עשר נטיעות ערבה וניסוך המים הלכה למשה מסיני (דנייאל ד) אשתומם כשעה חדא ואמר שכחום וחזרו ויסדום ומי א"ר יוחנן הכי והא"ר יוחנן דלכון אמרי דלהון היא ל"ק כאן במקדש כאן בגבולין

Rav Yochanan says that the minhog of Hoshannos was forgotten and the nevi'im reinstated it. Rashi there says "ahl pi hadibbur." The Rashash and the Maharitz Chajas both express their perplexity at this Rashi, because the Gemara in Temura 16 (regarding Asni'el ben Knaz) clearly states that when halachos are forgotten, a navi cannot reinstate them on the basis of his prophecy because the Torah is not Bashamayim. Neither offers an answer that explains Rashi.

A member of my shiur, Rabbi Gary Schreiber, said a brilliant answer which I think is 'karov l'emes.'

The Nevi'im did not re-instate the halachah as a d'oraysa. They were told through Nevu'ah that they should institute a new din d'rabbanan that happens to replicate what was a din d'oraysa. Despite the fact that they, and we, knew that this had previously been a din d'oraysa, the rule of lo bashamayim mandated that it be treated as a din d'rabbanan forever after. This explains why Rav Yochanan referred to it as a Yesod Hanevi'im.

The great accomplishment of Asni'el ben Knaz was that through his pilpul, he was able to darshen the dinim that had been forgotten from the Torah itself with the rules of drasha, and therefore they remained d'oraysa, just like kidushei kesef is d'oraysa.

UPDATE: Eight years after posting this, Reb Shalom Weiner sent in an intriguing idea.  He says that the din of lo bashamayim only applies to such halachos that are implicit or derivable from the Torah she'biksav.  But Halacha l'Moshe MiSinai was never covered by the rule of LBH.  In his words:

I claim that when Rashi says “al pi ha-dibbur” on Sukka 44a, he does indeed mean that the law regained its full original authority as halacha le-moshe mi-sinai. My theory is that while the rule lo bashamayim hi (as in Temura 16) clearly applies to laws derived via methods of drasha and sevara (e.g. 13 middos of Rebbi Yishmael), it may not necessarily apply to halacha le-moshe mi-sinai.

Lo bashamayim hi empowers chazal to use their Torah-intellect to determine through analysis, debate, and majority consensus what the Torah meant. It is not legally possible to use nevu’ah etc. in this sphere. Whereas for a halacha le-moshe mi-sinai, the role of Chazal is different – they are simply preserving the revealed law. Such laws needn’t make any sense to them, and their authority isn’t dependent on logic or sechel ha-yashar at all.
So if a halacha le-moshe mi-sinai is temporarily forgotten, it is reasonable that if a navi reminds us later, then that law should recover its full authority. All that is required is merely a gilui milsa. Whereas when we forget laws that were not received by Moshe as halacha le-moshe mi-sinai, but that instead were taught by Moshe as regular dinei Torah subject to principles of drasha and logic etc., then that’s precisely where lo bashamayim hi clearly applies. A navi cannot hand down authoritative drashos/sevaros through nevu’ah, and so prophecy could not properly resolve Yehoshua’s s’feikos or recover lost drashos, just like a bas kol could not convince Rebbi Yehoshua to agree with Rebbi Eliezer.
This distinction is analogous to Rambam’s famous discussion in the introduction to Peirush ha-Mishna about halacha le-moshe mi-sinai not being susceptible to machlokes. Because Chazal’s role is fundamentally different and more limited for halacha le-moshe mi-sinai – they are simply preserving the previously-revealed law, not determining it – it seems reasonable that lo bashamayim hi might not apply to such laws in the same way that it does for drashos/sevara.
Is this distinction consistent with Temura? Looking carefully, it seems to me that on Temura 16, only one amora – Rav Yitzchak Nafcha – applies lo ba-shamayim hi (etc.) to a halacha le-moshe mi-sinai (i.e. chatas she-mesu b’alav or nischapru b’alav). The other statements cited there mostly refer generally to “halachos” being forgotten, which could simply mean laws, not necessarily halachos le-moshe mi-sinai. In fact, the final braysa (in connection with which Asni'el ben Knaz is mentioned) refers specifically to drashos, and also to dikdukei sofrim. None of the examples given by Rashi (or in Shekalim) for dikdukei sofrim are halacha le-moshe mi-sinai.
So what Rashi says on Sukka 44a is consistent with Temura 16 according to my suggestion, except for Rav Yitzchak Nafcha who evidently holds (I admit) a somewhat different understanding of lo ba-shamayim hi. But there is no indication that Rav Yitzchak Nafcha is the consensus view. Furthermore, even if we believe Rav Yitzchak Nafcha is correct, the Gemara in Sukka that Rashi explains – which is only a hava amina about what Rav Yochanan meant – doesn’t have to be harmonized with Rav Yitzchak Nafcha’s view.
According to my approach, the question of whether lo ba-shamayim hi applies to halacha le-moshe mi-sinai remains somewhat open: According to Rav Yitzchak Nafcha in Temura it does apply, but according to other views in Temura it may not, and according to the hava amina in Sukka 44a per Rashi it does not.
I have two questions on the approach suggested in your 2006 posting:
Why does Rashi bother to say al pi hadibbur? If the law was only de-rabannan after it was restored, why does it matter if this was done al pi hadibbur or not? Of course Rashi isn’t just telling us a story.
If the hava amina on Sukka 44a really means that the ritual of arava be-mikdash became de-rabannan, then why did Rav Yochanan refer to arava in his statement as a halacha le-moshe mi-sinai in a single group together with nisuch ha-mayim and 10-netiyos? According to this view, throughout bayis sheni actually arava did not have the status of halacha le-moshe mi-sinai, and so it was not like the other two halachos Rav Yochanan grouped it with.
I admit these aren’t unanswerable. But I claim my approach is smoother:
Rav Yochanan called arava a halacha le-moshe mi-sinai and grouped it together with two other such halachos in his statement, precisely because the law did have full status as a halacha le-moshe mi-sinai after the nevi’im reminded us al pi hadibbur. Rashi therefore specifically says al pi hadibbur, precisely so we can understand that the law was restored to full status as a halacha le-moshe mi-sinai like the others that Rav Yochanan mentions, and not just a takana de-rabbanan.