Most of the x1000 high-voltage oscilloscope probes I find seem to have input impedance around 100M Ohm. For example the P6015A from Tektronix (100M Ohm) or the PPE6KV from LeCroy (50M Ohm) or these from North Star (100-400M Ohm). I thought that the passive attenuating probes just have a resistor in them (and a capacitor) which forms a voltage divider with the oscilloscope's internal resistance of 1M Ohm, so I'd expect a 1000M Ohm input impedance for a x1000 probe.
They all use 50 Ohm cable for optimal BW of >50MHz thus Rin is 50MOhm to 400M with (?) 2 stage dividers. Using 1Gohm requires active FET probes rated only for 25V or so.
It would also become unsafe at hiV with creapage going to insulator resistances of 1GOhm. Leakage would cause Partial Discharges. (faults)
Why would you want 1GOhm for high voltage in a tiny probe?
Use C ratio divider for HVAC. Then use a divider with 50 Ohm termination.
2000:1 probes use 93 Ohm coax to reduce capacitance.