I looked at TSRemapOSResponse. This is a remap callback that is invoked synchronously in HttpTransact::handle_response_from_server() and gets passed a ServerState_t value.
Note that there is no TSAPI analog for ServerState_t.
The main difference between TSRemapOSResponse is that connection retries will be visible to TSRemapOSResponse. It's not that clear to me whether we have enough API for that to be interesting.
This ticket is to investigate the utility of TSRemapOSResponse. If it is really helpful, then we should support it as a proper hook (global and transaction). If it's not then consider removing it.