Devarim (04:02)You shall not add anything to what I command you or take anything away from it, but keep the commandments of your G-d that I enjoin upon you.
Rashi (Devorim 04:02)YE SHALL NOT ADD — For instance, to place five chapters in the Tephillin, to employ five species of fruit and plants in the fulfilment of the command of Lulab And to place five fringes on one’s garment. Thus, too, must we explain the following words ולא תגרעו, Ye shall not diminish [from it]" (Sifrei Devarim 82:4).
Ramban (Devorim 04:02) However, the prohibition against adding [to the Torah] by word of a prophet we derive only from the verse stating, These are the commandments, which establishes, “From now on, no prophet is permitted to originate anything [in the Torah].” Whatever [laws] the Sages have established in the nature of “a fence [around the Torah],” such as the secondary degrees of forbidden marriages — that activity of [establishing fences] is itself a requirement of the Torah, provided only that one realizes that these [laws] are a result of a particular fence and that they are not [expressly] from the mouth of the Holy One, blessed be He, in the Torah.
Vayikra (27:34)These are the commandments that G-d gave Moses for the Israelite people on Mount Sinai.
Chizkuni (27:34)“these are the commandments;” no future prophet has the authority to either add to them or to cancel any of them. (Sifra)
Modern belief is that if a godol or tzadik tells you that something is halacha - it must be accepted even against a clear mesorah. This perhaps started with the belief that one can not disagree with the Chazon Ish.
Rav Moshe Feinstein: Can one disagree with the Chazon Ish & other gedolim
This view is still clearly alive. My son went to a major posek in Bnei Brak for a haskoma to his sefer. which he got but it included the caution "I can not agree with his halachic conclusions that result from his analysis because in many issues they conflict with the rulings of the Chazon Ish"
Similarly I have been criticized for disagreeing with the halachic views attributed to Rav Shmuel Kaminetsky and Rav Nota Greenblatt
In other words the view of gedolim must always be accepted while that of a prophet is disregarded if it goes against the halacha.