Regarding the special attenuator substrate:
I don't believe that polyethersulfone sees very much use these days. I do
a certain amount of microwave PCB design and I can't say that I've ever
even seen it discussed in the modern literature, or listed on a fab
shop's capabilities list. Of course, with enough money some shops will
build you whatever sort of science project you want (not to say that PES
is/was a science project).
Hook is an effect caused by a change in Er across frequency. Fourier then
says that your microstip (etc) impedance will vary at different locations
in a square wave, and you can end up with some goofy-looking pulses. And
swept signals will have varying amplitudes/losses/etc.
Here in the 2000's, where we're quite enamoured of extreme
digital signalling rates, we have a lot of substrates that are designed
specifically to avoid this problem. As far as I can tell, pretty much
anything better than 'good old FR4' (or its RoHS cousic 370HR) will be fine
for any 'exotic' 7000 time domain plugin use.
I could be wrong, this is not legal advice, YMMV, etc.