Received this perplexing rookie error while trying to undertake maintenance on one of my virtual servers.

An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.matrix.org/debian bullseye InRelease: The following signatures were invalid: EXPKEYSIG F473DD4473365DE1 matrix.org packages packages@matrix.org

Apparently apart from running the usual such as…

/usr/share/keyrings$ sudo wget -O /usr/share/keyrings/matrix-org-archive-keyring.gpg https://packages.matrix.org/debian/matrix-org-archive-keyring.gpg

…I needed to also hop into /etc/apt/sources.list and comment out the line…

deb https://matrix.org/packages/debian/ focal main

e.g From…

# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse
deb https://matrix.org/packages/debian/ focal main
# deb-src https://matrix.org/packages/debian/ focal main

…To…

# deb-src http://security.ubuntu.com/ubuntu focal-security multiverse
# deb https://matrix.org/packages/debian/ focal main
# deb-src https://matrix.org/packages/debian/ focal main

After that, was able to upgrade my Matrix Synapse homeserver without error