$A_3$-formality for pro-2 Demushkin groups

We study a weak form of formality for differential graded algebras, called $A_3$-formality, and show that the differential graded ${\mathbb{F}}_2$-algebras of continuous cochains of all pro-$2$ Demushkin groups are $A_3$-formal. We prove this result by an explicit computation of the Benson–Krause–Schwede canonical class using the classification of pro-$2$ Demushkin groups by Demushkin, Serre, and Labute. Compared to the case of odd primes, the new idea is to interpret the data of the canonical class as defining systems of higher Massey products.