The length of HDMI 2.1 cables was limited to a maximum of a few meters, but with the new 2.1a standard, that is about to change dramatically.
The very high throughputs associated with HDMI 2.1 data create physical limitations on cable length. To use cables longer than a few meters, an HDMI amplifier is needed. Soon, no more, thanks to the HDMI 2.1a standard. This includes active cables, which are powered from the HDMI connection itself. So no separate power supply or amplifiers are needed, which is always nice. However, the solution is one-way. In other words: it is clearly indicated which side of the cable must be plugged into the ‘transmitter’ and which into the ‘receiver’. Also, the source must be able to handle 2.1a, otherwise it will not work. Currently there are no cables or sources able to do so. But that is undoubtedly going to change soon now. And there is of course no need to give up completely because you own an ‘old’ source: there are always the current solutions available.