The Tastes of the Mitzvot

Hello Rabbi, let me just start by acknowledging your gratitude for all the lessons that the Rabbi uploads to YouTube, as well as for all the books that I have read, many of which I have learned a great deal thanks to you.
I wanted to ask what the ruling is in the case where the reason for a mitzvah is nullified from the Torah, is the mitzvah nullified? Suppose if Maimonides gives a reason for a mitzvah and this reason does not exist today, is the mitzvah nullified, at least according to his method, or do they say that the reason is nullified, does the reason not nullify the regulation, as they say in the case of mitzvot from the rabbis? I need some evidence for this.
And of course, the question is in a case that is certainly devoid of meaning, and not as we noted in Solomon who said, "I will increase and not diminish."
For example, in Parashat Reh, the teaching on the reason for the commandment of slaughtering: "And it is further stated that the reason for slaughtering from the neck with a sharp knife is so that we do not harm animals too much, because the Torah permitted them to man for his own benefit, to feed on them and for all his needs, and not [in order] to harm them gratuitously; and the Sages have already spoken extensively about the prohibition of 'harming animals'… and they have argued that it is a prohibition of the Torah."
If we assume that today the commandment of slaughter causes a lot of suffering to the animal, does he think that slaughter should be done using a different technique, or should he seek a different reason for the Torah commandment?
This is just an example, but I would be happy if the answer was not about the example, but rather a general reference to canceling the commandment of the Torah.
Thank you very much and good Saturday.