I'm sure they already do use, e.g. work experience as a signal.
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 ...
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