A finitely generated ICC non property (T) group that is neither inner amenable nor properly proximal

This note record a finitely generated extension of Caprace’s example of a non-inner-amenable group is also not properly proximal, which answers the original question by Boutonnet, Ioana and Peterson. The extension is an explicit ICC subgroup of ${\mathrm{GL}}_4({\mathbb{F}}_2(t))$, which has no property \textup{(T)}. Failure of proper proximality follows from the work of Ishan-Peterson-Ruth and the passage of proper proximality to co-amenable subgroups. Non-inner-amenability follows from a property-\textup{(T)} subgroup and an elementary centralizer computation.