That should be indicated by the Content-Type header, not the status code. If you get a 2XX response but the Content-Type isn't what you expect, you probably shouldn't try to parse it. I've seen misbehaving APIs return 2XX when they really should've returned 503.
Often this it to get around the inability for some cloud-based load balancers to accept 5XX status codes as healthy responses. Take AWS ELB/ALB, for example. There are conditions under which a target instance knows the underlying issue isn't related to its own health, such as during planned maintenance, upgrades, or database failures. In these situations, it would be desirable to return 503 and configure ELB/ALB to accept that as a healthy response. Since AWS won't let you do that, some applications will just return an empty or bogus 200 response during upgrades or maintenance.
You have to validate that the response is well-formed even if you parse it. There's no harm in trying to parse it if the Content-Type indicates it's JSON (or XML, or whatever else you're expecting). You can then use that result--or lack thereof, in case you couldn't parse it--to determine why you got a particular status code.
If a resource isn't found for any reason, 404, 410, or 451 is the correct response. If you want to clarify why it's not found, that should be included in the response body. Don't return 200 while simultaneously reporting an error--that's just bad form. 2XX means everything is good, 4XX means problem on my end, 5XX means problem on the API's end. It's an easy way to tell at a glance who's likely at fault. Yes, status codes are always going to be ambiguous, but that's why there are response bodies alongside them. If the Content-Type header is something you recognize, you can at least attempt to automate that disambiguation process.
Often this it to get around the inability for some cloud-based load balancers to accept 5XX status codes as healthy responses. Take AWS ELB/ALB, for example. There are conditions under which a target instance knows the underlying issue isn't related to its own health, such as during planned maintenance, upgrades, or database failures. In these situations, it would be desirable to return 503 and configure ELB/ALB to accept that as a healthy response. Since AWS won't let you do that, some applications will just return an empty or bogus 200 response during upgrades or maintenance.