In the US, those are protected categories about which a prospective employer may not inquire. This point was explicitly called out and repeatedly reiterated in the training I received a few years back about how to participate in an interview from that side of the table.
Some of that information can be obvious, as in the case of visible disability and occasionally ethnicity. But it's not permissible to inquire in any case. And speaking from the side of the interviewee, such questions would immediately bring the interview to a not overly friendly close, both because the answers are none of any prospective employer's business, and because no company so insensate to liability concerns is likely destined to end up anywhere I care to go.
Unless the protected characteristic is in some material way relevant to the role, asking such questions is likely to contravene the law or laws establishing the protection. It's not a criminal act, but it is unlawful; to do so, without a very clear and compelling argument for why it's relevant to the candidate's ability to perform the job, places the organization at the mercy of any candidate who cares to file a claim.
This is false, it is generally not illegal to ask about protected characteristics, it's only illegal to use them in hiring decisions. The distinction is important.
The reason people avoid asking (and tell others to avoid the same) is to avoid accusations or the appearance of using that sort of information in their hiring decision. You really have no defense if you ask about ethnicity and then decide not to hire that person, it's logical that you were asking about ethnicity because you wanted to use that information in your decision.
"Don't ask don't tell" is a good policy.
Once you actually have the job it's STILL just as illegal to use this information in promotions/firing decisions. However, it comes up in casual conversation all the time. People don't fret about asking a subordinate where they were born in casual conversation.
Fair. I sort of assumed from "don't even think about doing this unless you have signoff from general counsel in writing" that it was the latter, but on further research, I am indeed finding the former is more accurate to fact.
> In the US, those are protected categories about which a prospective employer may not inquire
No, they aren't. First, it's questionable whether sexual orientation is a federally protected axis of discrimination, and, second, it's illegal to discriminate on protected axes (except where they are bona fide qualifications, or some other exception to the general rule exists), not to enquire on them (HR will generally avoid enquiry not necessary for a clearly legal purpose to mitigate the risk that that will be viewed as evidence of discrimination, not because it is illegal in and of itself.)
Some of that information can be obvious, as in the case of visible disability and occasionally ethnicity. But it's not permissible to inquire in any case. And speaking from the side of the interviewee, such questions would immediately bring the interview to a not overly friendly close, both because the answers are none of any prospective employer's business, and because no company so insensate to liability concerns is likely destined to end up anywhere I care to go.