After 10 years, the siduction repo GPG key suddenly expired yesterday. Most of us would need a workaround to get the updated siduction-archive-keyring. There seem two ways so far, either manually getting and installing the .deb or telling apt to trust the 'unsecure' repo once, both as root:
Nach 10 Jahren ist gestern plötzlich der GPG-Key vom Repo abgelaufen. Die meisten werden wohl eine der beiden folgenden Lösungen ausprobieren müssen, um das aktualisierte Paket siduction-archive-keyring zu erhalten, entweder .deb herunterladen und manuell installieren oder apt einmalig das 'unsichere' repo erlauben:
1) wget and install
wget https://packages.siduction.org/extra/pool/main/s/siduction-archive-keyring/siduction-archive-keyring_2021.09.16_all.deb && apt install ./siduction-archive-keyring_2021.09.16_all.deb
2) apt allow insecure
rm /var/lib/apt/lists/packages.siduction.org_* && apt -o Acquire::AllowInsecureRepositories=true update && apt install siduction-archive-keyring
(confirm with y)
Both are not well tested yet, VOLUNTEERS, PLEASE TEST THIS

and confirm here
Edit: ^^ 10 hours later both ways are reported to work ^^