There are no cables yet that are guaranteed to meet the HDMI 2.1 feature sets, period. Just because a cable has been tested for 48Gbps doesn't mean that it can successfully transmit the HDMI 2.1 features that require that bandwidth without any errors, regardless of the cable mfr claims. The biggest issue in testing and possibly validating cables for the HDMI 2.1 feature sets is that there are no consumer devices yet that have validated HDMI 2.1 chipsets installed, so it's very difficult to determine if the cable will perform as expected in a real world setting and not just in a sterile testing environment. Reputable cable mfrs are continually testing and tweaking their cables/chipsets so that when validated consumer devices are available, they are ready for the final test. It's coming, but not quite yet. Besides, source content that takes advantage of the full HDMI 2.1 feature sets doesn't really exist yet so there's still time.
HDMI.org states in their HDMI 2.1 documentation that the maximum cable length for passive cables will be 1m - 3m (3' - 9'). However, my guess is that to achieve full 48Gbps on a copper-only passive cable the wire gauge will be thicker, which introduces its own set of issues. Active cables will probably be the way to go with hybrid fiber cables being the most reliable, especially over longer lengths.
If you have easy access to your cabling, then I would just purchase a cable that meets the HDMI 2.0 hardware specifications now, and then once HDMI 2.1 devices and sources are in the wild, upgrade your cabling to one that has been shown, in the hands of consumers, to work as expected for the HDMI 2.1 feature sets. Ruipro will be coming out with their 8k cable (Ruipro8k) hopefully by December, which is being actively tested by an ATC for the HDMI 2.1 feature sets. However, being as it is an active cable, HDMI.org does not allow for "certification" yet like they do for the Premium High Speed cables (18Gbps) so the cables will probably be labeled as Ultra High Speed HDMI. Unfortunately I don't believe that HDMI.org has registered that name like they did for the Premium marketing, so cable mfrs (at least most of them) are using that term in their marketing and product description without offering the consumer any test data (laboratory testing) to back up their claims. If you are concerned with eARC and VRR, which are part of the HDMI 2.1 feature sets, they are possible on the HDMI 2.0 chipsets if the device mfr built-in the ability for a firmware upgrade. Some tv mfrs (LG) are claiming that their HDMI 2.1 tv's will be able to upgraded to full HDMI 2.1 at a future date so that's something to consider.
Install your cabling in a conduit if you don't have easy access (in-wall installation for example) because that will be the ONLY way to future proof your cabling.
The cable is just the data pipe. How well one can utilize the HDMI 2.1 feature sets will be determined by the HDMI chipsets in the source and sink ends.