It's illegal to use proxy metrics that impact a protected class more than others. This is to prevent companies from discriminating by hiding behind tests whose main purpose isn't to match on skills needed for the job but to filter out members of protected classes. An employer can prohibit the wearing of a turban if it is a legitimate safety hazard. They can't prohibit it if the purpose is to ensure no Sikhs get hired. What's the purpose of leetcode? Does it map directly to a job's primary task or is it a way of filtering out certain protected classes such as those of an older age?

Age (>40) and family status are definitely on the list as well.

I assume you are referring to United States employment law?

For others: https://www.eeoc.gov/age-discrimination

U.S. Equal Employment Opportunity Commission

Age Discrimination: <<The Age Discrimination in Employment Act (ADEA) forbids age discrimination against people who are age 40 or older. It does not protect workers under the age of 40, although some states have laws that protect younger workers from age discrimination.>>

Yes, but it's similar in Canada and Germany* too.

* German academia has some incredibly stupid rules related to time-since-graduation, but age per se is a protected class.

What's legal varies by location, but even if it's not illegal to discriminate against the other criteria in the GP's list, a company may consider those factors and try to avoid that discrimination anyway. After all, you want the best candidate, regardless of what their home life is like.