All services outage
At 19:55 UTC, all services became unresponsive. The DevOps team were already in a call, and immediately started to investigate.
Postgres was running at 100% CPU usage due to a VACUUM, which caused all services that depended on it to stop working. The high CPU left the host unresponsive and it shutdown. Linode Lassie noticed this and triggered a restart.
It did not recover gracefully from this restart, with numerous core services reporting an error, so we had to manually restart core system services using Lens in order to get things working again.