It appears HDMI 2.1 will incorporate a lot of confusion, at least when first introduced, along with valuable features. An equipment supplier can decide to support parts, but not all of the HDMI 2.1 if they chose. Most likely the division in support will be at the speed of the connection, that is 18GHz (3 x 6GHz differential pairs) which is the current HDMI 2.0 speed or 48G (4 x 12GHz differential pairs) which allows all of the HDMI 2.1 features to be implemented if desired. The following quote from the Wikipedia HDMI entry provides insight:
"For video resolutions and refresh rates up to and including 4K/60 Hz, the Dynamic HDR, Game Mode VRR and eARC features are supported with the existing HDMI cables. Higher resolutions and refresh rates than 4K/60 Hz, such as 4K/120 Hz and 8K/60 Hz, require new 48G cables."
The lower speed features such as Dynamic HDR and eARC are of most interest to me, so I'm fine with that, your mileage may vary. Gamers that want 4k/120Hz output and higher for future high performance monitors won't be as happy.
It is likely at a lot of currently configured AVR's and the like can support the 18GHz HDMI 2.1 features using existing hardware with firmware upgrades, and the existing Premium cables already support 18GHz. This means many 2018 AV models may support these features if the standard and verification testing are in place. Features such as eARC upgrades are already promised for some 2017 models. Will upgrades to existing equipment for a feature such as eARC be HDMI 2.1 certified or will the individual features just be described and the owner can decide, who knows? At that point it seems that the feature, not the certification would be important, the unit has already been purchased. It is hard to see why a vendor would pay to have non-current models certified even if they supply some fully functional HDMI 2.1 features.
Certainly new models will be certified, when the test standards are available and the testing companies are ready, which does not seem to be the case at present. 2018 models should be interesting. It must be a hard time to be planning products in this space.