It didn't sound like that's the type of signal you were talking about. (Of course, "using some other feature" really means "using some other feature of the kind you're talking about"). It's unusual as a signal because it can also, rightly or wrongly, be justified as a bona fide qualification. In contrast, signals such as living in a high income area or mowing lawns as a kid would probably not pass the test and would be readily considered discriminatory.

Another problem with some of the things that employers can use as signals is that using them as signals is overall bad for society. For instance, employers usually want to hire people who already have jobs, because of what having a job signals. But over a whole society, this leads to the existence of a chronically unemployed underclass, which would not happen if the unemployment was evenly distributed.