An update:
By default, wget, curl, apt-get update, etc., will all pull down a stale http://openresty.org/package/ubuntu/dists/focal/InRelease file. Normal browsers pull down the current file.
Sending a "Accept-Encoding: gzip, deflate" header will force the correct file to be downloaded.
This works:
curl -H "Accept-Encoding: gzip, deflate" https://openresty.org/package/ubuntu/dists/focal/InRelease --output InRelease
This does not:
curl -H https://openresty.org/package/ubuntu/dists/focal/InRelease --output InRelease
Because apt-get update does not send "Accept-Encoding: gzip, deflate" on its requests, it always gets the wrong file from your servers! Please fix this! Ubuntu 20.04 can't use openresty's repositories in this state!