What is the derating of a 1Mohm oscilloscope input?

[Fungus]

So now I get to look into what it would take to measure slew rate accurately...

Turn on the slew rate measurement...? 

[ballsystemlord]

Of all the parameters you could measure of an op-amp, yes, it looks like it's just that easy. But everything else, settling time, quiescent current, voltage offset, PSRR, and CMRR, are all more difficult.

[Someone]

You can calculate it based on that parallel RC element in series with the protection network.  I have done this before and gotten results pretty close to the safe overload specifications.  The protection diodes can handle up to a given amount of average current which defines the derating versus frequency.

In practice the input will withstand way more than 36 volts peak-to-peak at *any* input attenuator setting.

Having looked rather carefully at such matters I'd want to see the schematic (and part sizes) of the specific front end before waving away "any input attenuator setting". For the older 1054 it was an attenuator part that was the first limiting factor in frequency derating:
https://www.eevblog.com/forum/testgear/rigol-ds1202z-e-input-voltage-vs-frequency-derating/

Dave has already done a teardown. Here is the album and I think I found the frontend picture for you.
https://www.flickr.com/photos/eevblog/albums/72157701904033821/with/45805998381
https://www.flickr.com/photos/eevblog/45805998381/in/album-72157701904033821

Why for me?

Because you literally just said you wanted to see, "the specific front end..." so now you can see it.

So where is the schematic?

[ballsystemlord]

You can calculate it based on that parallel RC element in series with the protection network.  I have done this before and gotten results pretty close to the safe overload specifications.  The protection diodes can handle up to a given amount of average current which defines the derating versus frequency.

In practice the input will withstand way more than 36 volts peak-to-peak at *any* input attenuator setting.

Having looked rather carefully at such matters I'd want to see the schematic (and part sizes) of the specific front end before waving away "any input attenuator setting". For the older 1054 it was an attenuator part that was the first limiting factor in frequency derating:
https://www.eevblog.com/forum/testgear/rigol-ds1202z-e-input-voltage-vs-frequency-derating/

Dave has already done a teardown. Here is the album and I think I found the frontend picture for you.
https://www.flickr.com/photos/eevblog/albums/72157701904033821/with/45805998381
https://www.flickr.com/photos/eevblog/45805998381/in/album-72157701904033821

Why for me?

Because you literally just said you wanted to see, "the specific front end..." so now you can see it.

So where is the schematic?

Umm, the way Dave went through it on the video, I thought the layout was all visible in the pictures. I could try and draw one up if you really wanted one.