The problem has been fixed, Signal notifications are being delivered again.
What happened:On April 1, 13:30 UTC, our signal account became unregistered. This was caused by a new Signal protocol feature – SPQR (Sparse Post-Quantum Ratchet) – becoming mandatory and our Signal client software not supporting it.
The fix:Updated to the latest signal-cli version (0.14.1), registered a new account with the same old number, and re-added the linked devices.
For more background information,
here is an issue on the signal-cli issue tracker discussing the account de-registrations on April 1.