Are all stable Xenes (graphene-like 2D honeycomb sheets) buckled?
According to the authors of Ref. 1 this is a general property. They provide an explanation for the stability of such sheets based on formation of particular buckled geometries: 


  The discovery of a flat two-dimensional crystal known as graphene has contradicted Landau−Peierls−Mermin−Wagner arguments that there is no stable flat form of such crystals. Here, we show that the “flat” shape of graphene arises due to a microscopic buckling at the smallest possible interatomic scale. We show that the graphene, silicene, and other two-dimensional crystals are stable due to transverse short-range displacements of appropriate atoms.


This explanation is corroborated by experimental observations:


  On the other hand, the generality of our prediction of the buckled universal shape has been revealed in recent experiments on silicene [...]


The authors note in the introduction that their explanation is only one among three that at that time were being seriously considered. The two other arguments are that (1) the sheets are trapped in a quenched metastable state that is not subject to large thermal perturbations compared to the lattice strength and (2) the sheets are slightly warped in the third dimension, the deformation suppressing thermal excursions.

The reference cited above is from 2012. Low-buckled 2D lattices have been reported more recently, for instance for germanene in Ref. 2.

Reference


A. O’Hare, F. V. Kusmartsev, and K. I. Kugel .  A Stable “Flat″ Form of Two-Dimensional Crystals: Could Graphene, Silicene, Germanene Be Minigap Semiconductors?
Nano Letters 2012 12 (2), 1045-1052. DOI: 10.1021/nl204283q 
Jincheng Zhuang, Nan Gao, Zhi Li, Xun Xu, Jiaou Wang, Jijun Zhao, Shi Xue Dou, and Yi Du . Cooperative Electron–Phonon Coupling and Buckled Structure in Germanene on Au(111). ACS Nano 2017, 11 (4) , 3553-3559. DOI: 10.1021/acsnano.7b00687.