Novak Djokovic probably played his final grand slam event this year because the Serbian will not get vaccinated and the US most likely won't allow him entry.
Novak Djokovic won his 21st grand slam trophy yesterday at Wimbledon but it's unlikely he'll match Nadal for the 22nd because he probably won't play at the US Open. Nothing is set in stone obviously but as the situation currently stands, we won't see Djokovic and Flushing Meadows.
The reason is quite simple. The Serbian has reiterated his commitment to not getting vaccinated fully aware of what that might mean for his legacy and tennis in general. The US does not allow unvaccinated foreigners to enter the country which means unless that stance changes, Djokovic won't be able to enter the country by any legal means.
There were talks of a possible exemption but the US Open shut down those talks rather quickly opting to follow the guidelines set out by the US government.