Suggestion: create a healthCheck endpoint that is unaffected by disabling the services directory.
Currently, the documentation recommends configuring a load balancer's health check to point at <server>/rest/info/healthCheck to determine if:
the responding machine in the ArcGIS Server site is able to receive and process requests. For example, during site creation, this URL reports the site is unhealthy because it can't take requests at that time.
Unfortunately, disabling the Services Directory also disables ALL html requests to rest/info, including those to the healthCheck endpoint. This prevents administrators from effectively using the healthCheck endpoint with a load balancer in a production environment without also leaving the Services Directory enabled.