If this is talking about a "placeholder" index.htm that comes with the server, I think the answer is obvious: If you request that page, it will successfully find and serve it in the same way as any other page. Thus 200 is the expected response.

On the other hand, if there's no page to be served, not even a default one, then it should 404. (Unless the default config is to list the (empty) directory, and it exists, in which case 200.)

There's no need to complicate things beyond that. One thing that I've learned over many years of experience with software is that if at all possible you should never add additional conditions or edge-cases, because they will tend to create more problems than they solve. The server is behaving most consistently if it treats the placeholder page the same as any other.

Would you want a compiler that specifically detects "hello world" programs and compiles them to always return failure, under the similar argument that it's not a "real" program? Because that's the logical conclusion of this sort of inane overthinking.