Leave it to the International Air Transport Association (IATA) to always deliver. Also, it happens to be really helpful that the IATA works directly with multiple airlines around the world and has strong ties to different countries.
Unbeknownst to most, the IATA has published an interactive map of COVID-19 related travel regulations around the world. The map is powered by Timatic, the software that airlines use when it comes to validating passenger travel requirements and complying with border regulations.
Overall, the best resource is the official government tourism website of a destination. Though, if you're not completely fixed on a particular destination, the IATA Timatic map might be the best place to start narrowing it down.
For the most up-to-date and accurate information, the IATA map is certainly going to be the best bet, although there are certain places where it fails to cover ground since things are still changing. For example, Alaska and Hawaii have separate restrictions aside from the U.S. mainland, which is not noted in the map at all.
(By comparison, the KAYAK map seems like it was updated ix days ago. That's like a century ago.)
Of course, take everything from a grain of salt unless it comes straight from the horse's mouth.
