Some Mitzvos do not apply anymore because they were only temporary in nature meant for a specific time. For example, there was a temporary prohibition against anyone but Moses ascending Mount Sinai, but that Mitzvah no longer applies in current times because it was only in effect during the time that Israel was camped there. The same is true concerning the commandment of putting blood on one's doorposts (Exodus 12:23), erecting stone monuments with the words of the Torah written on them (Deuteronomy 27:1-8), and of the prohibition of intertribal marriage (Numbers, Chapter 36). Maimonides writes in Root 3 of his Sefer HaMitzvos that he only included everlasting mitzvos in his enumeration of the 613 commandments, but temporary mitzvahs are still considered mitzvahs despite their temporary nature. Rabbi Gershon Edelstein (Rosh Yeshiva of Ponovezh) says that the Rashbam (to Bava Basra 111b) maintains that the prohibition of intertribal marriage still applies nowadays, but is not practically applicable because a stipulation in the commandment says that it is only a prohibition for those of the generation which left Egypt in the Exodus. However, the Rashbam writes, if it was possible that someone from that generation was still alive now, that person would still have the prohibition. Perhaps one can say that all temporary commandments were not necessarily temporary commandments, but were commandments that were decreed forever but were only commanded to people of specific generations. Maimonides (in Sefer HaMitzvos) writes that perhaps one can say that the Mitzvah to destroy the seven nations is no longer in effect. Based on the above, perhaps the understanding of this opinion of Maimonides could be that the commandment itself is technically still in effect, but that the people to whom the commandment applies (whether those are supposed to kill them or those are supposed to be killed) no longer exist, so for all practical purposes the commandment is gone. However, in order to say that a specific commandment or prohibition was only for a certain generation or situation always requires an implication to that effect from the words of the Torah, one cannot just make up a new idea that a certain element of the Torah does not apply any longer.