At around 2025-06-23 02:50Z we went through our normal Monday GKE maintenance window which resulted in a rolling node upgrade.
It appears that the elasticsearch pod were then unable to come up on the new node due to hitting Init:ImagePullBackOff for this image docker.io/bitnami/bitnami-shell:11-debian-11-r123. This is used for an init container.
Initially I assumed this was due to hitting the new docker rate limiting but actually it was due to the image totally disappearing from dockerhub. Presumably after bitnami renamed the repo (which admittedly they do describe as deprecated).
The resolution to this was changing the image name to bitnami/bitnami-shell-archived to point at the newly renamed archived repo.