Insofar as HDMI 2.1 is concerned, the only relevant spec for AVR manufacturers will be eARC (thus, Dolby Atmos), which can be added through a software update. AVR Manufacturers have been pushing out software updates for Dolby Vision, there's no reason to believe they won't also for eARC. People who buy a new AVR now (like I did) will likely be fine.
HDMI 2.1 is a little different than past versions since it is allowed to be "cherry picked" by manufacturers for the specs they need or don't need. Everything else besides eARC is either not here yet (4K 120Hz), way off into the future (like 8K passthrough), or not relevant to AVRs. I assume this is why Apple is holding off a bit on Dolby Atmos, since they want to implement eARC along with it which makes sense.