The 200GBASE-CR4 standard  defines ethernet communication with 200 GBit/s using four lanes and PAM4 modulation over a maximum distance of 3 m.
A single PAM4 symbol encodes two bits, therefore, I expect that a single lane is modulated at approx 25 GHz.
According to , 40GBASE-T is the Ethernet standard with the highest bandwidth using twin-axial copper cables. It uses four lanes, therefore, we have 10 GBit/s per lane and PAM16 encoding. Thus, there should be four bits per symbol and I expect that a lane is modulated at around 2.5 GHz.
From my understanding, such high frequencies are very difficult to handle as the wavelength becomes comparable to the dimensions of the cable and one has to use coaxial cables or other waveguides.
Particular problems I would expect at these high frequencies are high attenuation because of the skin effect  and an impedance mismatch due to thermal and mechanical effects on the cable.
- Is the 200GBASE-CR4 standard, communication of 200 GBit/s through twin-axial copper cable achievable?
- What effects make it difficult to transmit such high-frequency signals in practice? Are there any books which cover this particular topic?