Is "being illegitmate" still a legal concept?

In old mystery stories, illegitimate children do not inherit, and have other legal problems. No legal need for parental support, I suppose.

Now it seems that many couples never marry. Are their children missing out on any legal protections?

IANAL but children, regardless of their degree of ‘legitimacy,’ can inherit from their parents and receive child support. Women receive child support from men who are not their ex-husbands. Whether the couple is married or not even speaking to each other, the child’s welfare is their responsibility.

Illegitimacy can be used on the state level to restrict inheritance rights. The Supreme Court held in Labine vs Vincent that states may exclude illegitimate children from inheriting.

Recent thread on topic: Does “legitimacy” have any meaning outside of European title inheritance?

And across the pond British law prevents any child born out of wedlock from inheriting a title (Lord, Duke, etc) with one exception. Under Scots law an illegitimate child (adulterines excluded) who’s parents marry after the birth can be legitimated and inheirit titles in the Peerage of Scotand

Whether or not it’s still a legal concept, I can say I’ve met some real bastards in my time.

I believe some jurisdictions rule that a woman’s husband is the legal father of any children she has for inheritance purposes, regardless of the possibility that the children’s biological father might not be their mother’s husband.

In order to claim Canadian citizenship, I had to prove that I was legitimate.

This might be an indirect consequence of legitimacy only. If you claimed Canadian citizenship on the grounds of Canadian descent (because your father was a Canadian), you would have to provide proof to the competent authority that you are indeed the child of a Canadian. Many jurisdictions include the concept of “fatherhood presumption” - the man who was married to the child’s mother at the time of birth (or conception, or both, or either one - YMMV) is presumed to be the father unless proven otherwise. Thus, it can be necessary for an applicant to provide evidence of his or her parents’ marriage in order to claim citizenship, without legitimacy being a strict prerequisite of citizenship.

In my case, my mother was the Canadian, but I assume it’s easier for the government to collect excess data than to collect only that which they need.