The only feature sets for HDMI 2.1 that are currently available are eARC and VRR. Both of which are capable with HDMI 2.0. There are no devices or content that can take advantage of the complete HDMI 2.1 feature sets yet so don't expect the higher video standards to be supported for quite some time. The 48Gbps bandwidth is mainly for the higher video standards so even if you find a cable that has actually been validated for 48Gbps, it's sort of meaningless at this juncture.
The cable is just a data pipe. It can not modify the signal it is carrying. That is determined by the HDMI chipsets in the source and sink end so yes, it is best to have your devices all using the same HDMI hardware version for compatibility. HDMI is backwards compatible but only to the in-common feature sets.
There is no such thing as an "HDMI 2.0 cable", or HDMI 2.1 for that matter. A Premium High Speed HDMI cable, with the QR label for authenticity, has been tested and validated by HDMI.org to meet ALL of the HDMI 2.0 hardware specificatons. 25' is the maximum certifiable length for passive cables. HDMI.org does not allow for the certification of active cables by an ATC (Authorized Testing Center). Premium High Speed HDMI cables can certainly pass HD Audio (DTS-MA, Dolby TrueHD, lossless Atmos, etc) depending on the hardware at the source/sink end. ARC/eARC can have issues that are distance related, especially if your cable run is longer than about 25'.
Ultra High Speed HDMI (UHS HDMI) will be the correct term for the 48Gbps cables to distinguish them from the Premium cables (18Gbps) but that term is being widely used already for cables that have not been proven to meet the HDMI 2.1 feature sets. The reason is that testing and validating is still on-going and that there are no validated consumer devices yet that can utilize the full HDMI 2.1 features, let alone source material. Even if a cable has been proven to pass 48Gbps that doesn't mean that it can pass the higher video standards that require 48Gbps reliably and error free.
