For notifications, you can setup Health Checks as @sdayman mentioned from the Traffic section of your Cloudflare Dashboard that will notify you when an origin is un-reachable.
Additionally, if your Cloudflare DNS provides another A or AAAA record or your Cloudflare Load Balancer lists another origin in the same pool, Zero-Downtime Failover automatically retries requests to your origin even before a Load Balancing decision is made. Cloudflare currently retries only once for HTTP 521, 522, and 523 response codes. Zero-Downtime Failover is available on our Pro, Business, and Enterprise plans.
Take a look at this article for further reference: https://support.cloudflare.com/hc/en-us/articles/360037465932-Preventing-site-downtime#6epLHd3oJb13iY9R06WPjN