Let’s Encrypt will return a 503 when we are certain the infrastructure is unavailable- but this is only possible when our load balancers are still up and accessible. There will always be some maintenances where we make changes to our networking gear and our datacenter is essentially offline. This usually only affects Staging because we don’t have a secondary datacenter that we can fail over to. As a result, the errors returned to the users are from our CDN. We think this is ok because of how rarely we take Staging entirely offline and cannot serve a proper 503 response.
In production, we mostly do non-interruptive rolling restarts and rarely turn off all access to the API. On the occasions where do stop Production API access, we make sure to return a 503 from our frontends whenever possible and provide a maintenance notice on status page about the downtime.