GPG key expired
It seems the admin@f-droid.org GPG key expired a few days ago.
$ gpg --list-keys --verbose admin@f-droid.org
gpg: using pgp trust model
gpg: Note: signature key 7A029E54DD5DCE7A expired Sat 24 Apr 2021 06:23:54 AM EDT
pub rsa4096 2014-04-25 [C]
37D2C98789D8311948394E3E41E7044E1DBA2E89
uid [ unknown] F-Droid <admin@f-droid.org>
sub rsa3072 2014-04-25 [S] [expired: 2021-04-24]
sub rsa3072 2014-04-25 [E] [expired: 2021-04-24]
The key in the latest admin@f-droid.org.jar is the same.