What I don't get is how FL -90 can mean 1000 ft below FL 0.
Because that is what the altimeter would (misleadingly) indicate, having "unwound" below zero, i.e. below the level of the 1013hPa datum.
I suspect that it has now been removed from Annex 2 because, in practical terms, even in the maximum recorded pressure and with a transition altitude as low as 1500ft, it would just not happen that even FL-95 would occur above the TA.
2 s