I honestly had never thought hard about reversing the relationship and having workers pull. This point about no longer requiring health checks is a real "woah" moment. Thanks for expanding my mind!
1. It can be more tolerant of failures, since an unresponsive server doesn't mean failed requests.
2. Hooking up your server scaling system to HAProxy, etc. isn't the easiest thing in the world anyway.
it can't be known by the server itself, as (among many other reasons) the server can't know about network issues between itself and any upstream caller
it can't be determined by out-of-band health check queries, because those queries don't represent actual traffic, the simplifying assumption that they _do_ introduces many common failure modes that any seasoned engineer can speak at length about
health checks can be a nice additional signal on top of monitoring actual prod traffic, but they can't be used by themselves, they just don't capture enough relevant information
a server can return 200 OK to every health check query and 5xx to every production request, is that server up or down? hopefully clear it is down
similarly, if it returns 5xx to every health check query but 200 to every prod request, is it up or down? hopefully clear it is up
if you want to monitor the status of a server (or application) for internal purposes, that's fine, but that's (at best) supplementary signal for the decisions made by a load balancer, not something you can make load balancing decisions on in isolation