It's unsupported, because if you use it and something goes wrong, you're on your own. We never explicitly set out to make the webserver/webapps/main path automatically hosted - it's just a coincidence of the way we set up Jetty. Some future upgrade of Jetty may make that work differently, or we could explicitly gate the feature behind a config flag, or something.
In practice, because it's such an open secret, we'd probably take pains to prevent anything from breaking it upon such a hypothetical Jetty upgrade, but the reason we claim it's 'unsupported' is because you're basically relying on undefined Jetty behavior, not anything coded into Ignition deliberately.