Is cyclononatetraenylium antiaromatic?
Cycloenneatetraenyl cation, using the Greek root for nine, is certainly not antiaromatic.  A ring that large will in general be nonplanar unless strong aromatic coupling is there to drive a planar shape, and if the planar form is antiaromatic so much the better for nonplanarity.  By classical rules such a nonplanar form would be nonaromatic.

Whether this nonplanar form is Mobius aromatic is open to interpretation, or at least it seems that way given the twisted nature of the ring.  In any event Mobius aromaticity is unlikely to give the strong stabilization we see in the Huckel version because the p orbitals do not overlap perfectly.  Most likely the structure and stability of the ion  are more determined by sigma bond interactions (minimizing steric strain) than pi bond interactions.