> Subdomains can be passwords and a well crafted subdomain should not leak

Your comment is really odd to read I'm not sure I understand you, but I'm sure you don't mean it like that. Just to re-iterate the important points:

1. Do not rely on subdomains for security, subdomains can easily leak in innumerable ways including in ways outside of your control.

2. Security by obscurity must never be relied on for security but can be part of a larger defense in depth strategy.

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https://cwe.mitre.org/data/definitions/656.html

> This reliance on "security through obscurity" can produce resultant weaknesses if an attacker is able to reverse engineer the inner workings of the mechanism. Note that obscurity can be one small part of defense in depth, since it can create more work for an attacker; however, it is a significant risk if used as the primary means of protection.