We have many guards making sure that there are always at least two instances of the kube-apiserver. If we ever reach a single kube-apiserver and it causes disruption for the clients, other alerts such as KubeAPIErrorBudgetBurn will fire.
KubeAPIDown is here to make sure that Prometheus and really any client can reach the kube-apiserver, which they can even when there is only one instance of kube-apiserver running. If they can't or that availability is disrupted, `KubeAPIErrorBudgetBurn` will fire.
Comment 23058588 by Marcel Härri at 2023-09-19T06:57:07.949+0000

Mar 12 18:52:24.937 - 58s E namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/firing severity/critical ALERTS
{alertname="KubeAPIErrorBudgetBurn", alertstate="firing", long="1h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="5m"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 26m54s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 26m54s, firing for 0s:
Apr 18 09:32:44.037 - 1288s I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}
Apr 18 09:33:14.037 - 298s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
Apr 18 09:55:44.037 - 28s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 27m26s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 27m26s, firing for 0s:
Apr 16 09:31:28.850 - 1438s I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}
Apr 16 09:31:58.850 - 208s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 47m4s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 47m4s, firing for 0s:
Apr 15 21:29:52.366 - 2196s I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}
Apr 15 21:31:00.366 - 628s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}