These meters also have a feature that you won't find in modern DMMs, let alone bench multimeters: a very low input current of just a few pA. Together with the 10 Mohms input impedance, this makes for a very useable pA-meter.
A very high input impedance is present on both handheld and bench DMM's today. On bench meter you can usually switch between 10Mohm and high, on handheld meters it is often always present in the mV DC range (On some high end meters it is switchable).
I'm not sure what you want to express with that?
I was not talking about input impedance, but input (bias) current. And this doesn't change, whether you have some 10 meg resistor across the input or not.
If I claim that no bench DMM comes even close to the old Metex 4650, you may expect that I have measured dozens of them, from venerable Schlumberger 7150 plus up to Keithley 2001. The best of them have a bias current of some 30 pA, but none of them comes close to <5 pA like the old METEX does.
Input bias current is especially bad on the meters with alleged high input impedance, like >10 Gohm for most bench DMMs in the low voltage ranges.
And this is the trap for youngplayers: you might think you can use such a DMM like an electrometer (for low voltages at least), but that's not the case, not by a long shot.
I also have checked several more modern handhelds. The "high impedance range", if present, often is only some 1 Gohm, but the really important thing is the (more or less constant) input bias current anyway. None of the more modern DMMs I've come across so far had nearly as low a bias current that it would be able to compete.