In the US, asking just about anything is allowed. Certain information is generally unlawful to use in hiring decisions; though, so if you do ask it and don't fall into one of the exceptions where it is allowed to hire based on it, you probably want to both say up front and be able to demonstrate by evidence when challenged in court that it is (1) used for some specific purpose other than hiring decisions, and (2) separated from the identifying information after receipt so that it could not be used for hiring decisions even if those making that decision wanted to.
There is a popular myth that the things that are illegal to use in hiring decisions are illegal to ask, which isn't true (this probably comes as a slight distortion from managers receiving legal advice that those things should not be asked because asking them increases the risk to the company, which is true not because they are illegal to ask, but because asking produces the risk that a jury will believe a charge that they were used in hiring decisions.)
There is a popular myth that the things that are illegal to use in hiring decisions are illegal to ask, which isn't true (this probably comes as a slight distortion from managers receiving legal advice that those things should not be asked because asking them increases the risk to the company, which is true not because they are illegal to ask, but because asking produces the risk that a jury will believe a charge that they were used in hiring decisions.)