At least, that's my diagnosis.
Here's the pertinent line from an entry in my /etc/apt/sources.list.d/:
deb http://ppa.launchpad.net/wireguard/wireguard/ubuntu focal main
Here's the output of 'apt-get update':
--snip--
0% [Working] 0% [Connecting to debian.map.fastlydns.net
(2a04:4e4 Err:1
http://ppa.launchpad.net/wireguard/wireguard/ubuntu focal InRelease
403 Forbidden [IP: 91.189.95.83 80]
0% [Connecting to debian.map.fastlydns.net
(2a04:4e4 Hit:2
http://security.debian.org buster/updates InRelease
0% [Waiting for
headers] Hit:3 http://deb.debian.org/debian
buster InRelease
[...]
E: Failed to fetch
http://ppa.launchpad.net/wireguard/wireguard/ubuntu/dists/focal/InRelease
403 Forbidden [IP: 91.189.95.83 80]