I'm glad you know how to do that, since you can patent and sell your techniques, and they will make you rich.
Unfortunately "adding limiters" isn't that simple, of course.
Well, it's not rocket science. Of course it can be done. I have done it and scope vendors seem to have successfully done it as well.
So no chance getting a patent on that.
Limiters are non-linear circuit elements that will affect (i.e. distort) the signals even when they aren't limiting. In a scope distortion is to be avoided, which implies limiters might need to be avoided. Many scope and spectrum analysers are deliberately left "unprotected" for that reason - even though their lack means the instrument would be destroyed by too large signals.
Yes, for an instrument promising a 3rd order dynamic range of 70dB and operating up to several GHz, a limiter would be a challenge indeed. Not saying it cannot be done, but that's something I haven't tried before and would actually expect some degradation in performance.
But we are talking about general purpose 8-bit scopes in the couple 100MHz range here. With an INL of at least +/- 0.5LSB there's not much to worry about the linearity of the frontend.
Plus, of course, there are many amplifiers in a scope, any of which could be overloaded. "Protection" would require many non-linear elements in the signal path, which is even more problematic!
No, there aren't. Neither many amplifiers, nor particular non-linear elements.
Apart from that, I'm pretty sure even the Rigol DS1000Z will have some clamping diodes at the input - just clamping to the supply rails instead to a specific voltage level that prevents the amplifier from leaving its specified common mode range.