The danger is that the endpoint becomes public knowledge and comes under a DDOS attack. Putting an IP address filter on that endpoint is usually enough to stop that.
I'm aways up for hearing about other ways people solve health checks.
Emphatically agree, but it's important in the first place to design and deploy your infrastructure such that basic increases of scale are accounted for - prevention is the most important piece of the puzzle. Get that right and an overloaded server is symptomatic of something else, in which case taking down access to the unruly resource is first priority.
IMO, the big takeaway here is that they were load balancing simply by only hitting the top level - selectivity is somewhat tedious to build but worth it in the long run.
Why is there even direct external IP connectivity to the realserver, sidestepping the loadbalancer?