Some Mitzvot have Expiration Dates

How eternal is the Torah, really?

In Niddah 61b, Rav Yosef says

מצוות בטלות לעתיד לבוא

which the William Davidson Talmud translates as

mitzvot will be nullified in the coming days 

Except that some mitzvot already have been nullified.

Some mitzvot have explicit expiration dates. One obvious example is the rites of dedication of the mishkan, as described in Sh'mot chapter 29.  Another example is the prohibition of intermarriage among the tribes, in Bamidbar chapter 36, which, according to Rava in Bava Batra 120a, was in force for only one generation:

אמר רבא: אמר קרא (במדבר לו, ו) "זה הדבר" - דבר זה לא יהא נוהג אלא בדור זה. 

William Davidson Talmud translation:

Rava said that the verse states there: “This is the matter that the Lord has commanded” (Numbers 36:6), meaning: This matter will not be practiced except in this generation.

Indeed, RaMBaM does not include mitzvot that have explicit expiration dates among his 613.

Other mitzvot are nullified by historical circumstances. The clearest example of this is the nullification of the prohibition of marrying Ammonite and Moabite converts. Here is how that nullification is presented in Mishnah Yadayim 4:4:

בו ביום בא יהודה גר עמוני ועמד לפניהן בבית המדרש.

אמר להם, מה אני לבוא בקהל.

אמר לו רבן גמליאל, אסור אתה.

אמר לו רבי יהושע, מתר אתה.

אמר לו רבן גמליאל, הכתוב אומר (דברים כג), 'לא יבא עמוני ומואבי בקהל ה' גם דור עשירי וגו .

אמר לו רבי יהושע, וכי עמונים ומואבים במקומן הן, כבר עלה סנחריב מלך אשור ובלבל את כל האמות, שנאמר (ישעיה י), ואסיר גבלות עמים ועתודותיהם שושתי ואוריד כביר יושבים.

אמר לו רבן גמליאל, הכתוב אומר (ירמיה מט), ואחרי כן אשיב את שבות בני עמון, וכבר חזרו.

אמר לו רבי יהושע, הכתוב אומר (עמוס ט), ושבתי את שבות עמי ישראל [ ויהודה ], ועדין לא שבו.

התירוהו לבוא בקהל.

Sefaria translation:

On that very day, Yehuda, an Ammonite convert, came and stood before them in the Beit Midrash, and said to them, "What is my status with regard to whether I can enter [via marriage] into the congregation [of Israel]?" Rabban Gamliel said to him, "You are prohibited." Rabbi Yehoshua said to him, "You are permitted." Rabban Gamliel said to him, "The verse says, (Deuteronomy 23:4) 'An Ammonite and a Moavite may not enter into the congregation of the Lord, even to the tenth generation,' and so forth." Rabbi Yehoshua said to him, "And are the Ammonites or Moavites still [dwelling] in their own place? Sancheriv, king of Assyria, already arose and blended all the nations, as the verse says, (Isaiah 10:13) 'I have removed the borders of nations, and I have plundered their treasures, and like a great warrior laid low the inhabitants.'" Rabban Gamliel said to him, "The verse [also] states, (Jeremiah 49:6) 'And afterwards I shall return the captives of the children of Ammon,' and they are already returned." Rabbi Yehoshua said to him, "The verse [also] states, (Amos 9:14) 'And I shall return the captives of my nation Israel,' [and Judah], and they are not yet returned." They [the Sages, subsequently] permitted him [the Ammonite convert] to enter into the congregation.

The historical circumstance that nullified this prohibition was the Assyrian policy of population exchange that mixed all ethnicities.

Now for my agenda. A whole set of mitzvot that have been nullified by historical circumstances are all the mitzvot associated with sacrificial worship.Statistically, all Jews are mamzerim (see here and here), from which it follows that there no longer are any true kohanim.

So what was sacrificial worship doing in the Torah in the first place? I'm with RaMBaM on this one. You can read RaMBaM's take on sacrificial worship in The Guide for the Perplexed Part III Chapter 32 (which, unless you can read the Arabic original, you may as well read in English because the Hebrew version also is a translation). Sacrificial worship was supposed to be a temporary expedient that was destined to be outgrown. First, it was limited to a specific location and had to be performed by the members of a specific extended family. Next (and this is where I go beyond RaMBaM) the system was designed to gradually invalidate the members of that extended family. We didn't get the hint after the Babylonians destroyed the first Temple, so God had to send the Romans to destroy the second Temple.

So there is no point in building a third Temple.