If we wanted the Parser to have a dependency on ResourceLoader, I suppose we could consider logging a warning server-side if during a fresh parse an added module is not known to the registry, but that feels a bit messy.
Another way might be to add small number of bytes client-side for these cases by letting them be included in the RLSTATE object we have for style module names, and then making sure that mw.loader.state() logs warnings for unknown modules.
Yet another way might be to build on the footer script we have in ClientHtml for styleDeprecations, which also produces console warnings. This has the benefit of requiring zero code in the startup client itself, and no additional code in the <head> either. Instead, it would generate exactly the code that is needed without any unused abstractions.