I’ve been out of touch on this sort of detail, but US military helos test specs for the structural demonstration series of maneuvers always included demo to -0.5 G*. ( and no, I don’t know how the Bell guys accomplished this data point with a teetering rotor,, but God bless ‘em, they must have found a way )
For the US Army UTTAS competition the Boeing and Sikorsky competitors had an additional maneuver requirement added, which in part required holding 0.0 G for two seconds.
*As measured at the aircraft center of gravity, thus the cockpit Nz would be lower ( due to the pitch rate ) esp. in larger machines.