Source: akshay nair
nambUtiri-paternal-uncle-shrAddha
In Shankarasmriti, the code of law which became popular in Kerala with rise of Namboothiri tradition, It is stated that the eldest son in a family must perform Pitr Karma for all his paternal uncles as well.
It is since younger brothers of father mostly married into matrilineal castes like Kshatriya-Nairs and Ambalavasis, and they did not have any heir to perform Pitra Karma for them becuase children in these unions belonged to mother’s family and they performed Pitr Karma to their maternal uncles or karnavars instead of their fathers. Hence, the eldest son in a Namboothiri Brahmin household performed Pitr Karma to his paternal uncles along with his father.
The younger brother in a Namboothiri family was allowed to marry within community (i.e. Namboothiri women) only if the eldest brother did not have any son to pass in the paternal lineage.
śrautāgni
Shankarasmriti states a Brahmin should never abandon his Agni. If he does he will become equal to Shudra. Wonder how this rule worked for Nambuthiris who were into Tantrika tradition. Perhaps context of this rule was when Vedic Shrauta ritualism was widespread among Nambuthiris.
dvija-vēdādhyayana
Another thing is that Shankarasmriti forbids Vedadhyayana for Kshatriyas & Vaishyas. It states only a Brahmin has right to learn Veda in Kerala. Kshatriyas are initiated in Gayatri japa after upanayana, but they couldn’t learn any other Veda Mantras and are on par with Nairs.
paśubali
Shankarasmriti’s defense of pashubali: It is stated that animal sacrifices in Yajnas with guidance of Mantras is not himsa. However outside of ritual context one must never kill & eat animals. If one does, he will aquire sin equal to the total number of hair in animal’s body.
Vratyastoma
As per Shankarasmriti, the Dwijas who don’t undergo Upanayana at the right age will fall from their Varna and become Vratyas. Vratyastoma ritual is done to admit the fallen Dwijas back into their respective Varnas. But in Kerala, this ritual is invalid. Hence the theory that the Nairs are fallen Vratya Kshatriyas who couldn’t regain their Dwijahood and remained as Sat-Shudras deserve some merit.