British Airways has a new interactive travel tool where you input your travel information (Departing City, Connection City if Applicable, and Arriving City), your vaccine status, what country's passport you hold and the dates of your trip and it will give you all the covid restrictions you need to know. Upcoming rule changes in many countries around the world are also listed. It's actualy on a third party's website but it can be linked from the front page of the BA website.
The site is also available from the American Airlines website but the BA site offers connection information while the AA site doesn't.
https://apply.joinsherpa.com/map?affiliateId=americanairlines&from=bannerContainerA