"It's every man for himself" — is the idiom still the same or is there already a common genderly neutral option?

I would probably say “it’s everyone for themselves”… to me the other feels unnatural.

You can say either "it's everyone for themselves" and I think people will understand. The times I've heard it or read it, it was 'every man for himself' but I definitely would understand if someone said everyone.

Every person for themselves

Feminism hasn't yet reached "women and children first"...

To summarise what everyone else is saying in bits and pieces into a single answer: the idiom is common and very well-known. Most people wouldn't think twice about how they use it and it would just be seen as referring to everyone. But at the same time, you could substitute your own word and people would understand what you meant and wouldn't question it. It probably wouldn't even sound unusual to us (unless you decided to go really crazy and say something like "it's every space hamster for themselves" - and even then, people would just assume you were trying to be funny or weird).

You lose the idiomatic nature if you say “it’s everyone for themselves”. Same meaning, just not as punchy. “It’s a dog eat dog world” has similar implications.

I would say “It’s every person for themself.” But I wouldn’t yell at you for saying man.

Idiom is the same and is generally considered gender neutral when in use.

Man is considered gender neutral in a lot of expressions.

You could make it gender neutral by saying "every man for themself" thereby changing man to the more general meaning of human as opposed to male. See also mankind or man-made which are also technically ungendered.

People would understand your meaning, yeah. And it doesn't sound weird. But "every man for himself" is already gender neutral. "Man" was originally just the general word for a human, and a lot of old expressions still use it that way. 🙂 I don't think people started questioning it until the 20th century; I know I've seen an early 20th century author (I think it was G. K. Chesterton) say something like "men, and by that I mean specifically males..."

You could say that, sure, but it would be more natural to just say every man. Man is often used as a gender neutral word, as it’s referring to hu-man, not male

You could also say "every person" or, depending on context, "every student", "every employee", "every citizen", etc. Those are all a little bit closer to a direct analogue of "every man".

It depends on what you mean by "gender neutral". Because the "man" here could be used in the sense of human or mankind. A lot of masculine terms in English can be used in a gender neutral way because of how the language developed. You could think of the masculine term as gender neutral and the feminine version being a qualifier added to the masculine word. And in this case, "himself" is a grammatically correct pronoun. "Themselves" also works to emphasize gender neutrality, though it would make it ambiguous if it were singular or plural. "It's every man for themselves" is a version of the phrase I think I hear a lot.

You can replace most instances of gender neutral "man" with human. "It's every man for themselves" without gender becomes "It's every human for themselves" basically man = human

I always understood it as neutral anyways, "man" as in "mankind" which includes women.

People have been saying “everyone for themselves” for quite a long time

I know we all want to be gender neutral here, but I think there is (or once was) a cultural bit here that it _did_ mean every man for himself, which did not mean the men would leave women and children behind

It's such a self-explanatory phrase that any noun (e.g., dog, chicken, woman, child) will fit and be easily understood.

“(Let) the devil take the hindmost” is a gender neutral alternative that actually exists. A little old fashioned, but works.

In Scotland we substitute it with the c*** word, but I wouldn't recommend that. I don't think that in this case it particularly means man as in a male human. It's more like the man in mankind. You can use it for women. That said, I've heard 'it's everyone for themselves' to mean the same thing.

Well, we could always borrow "Sauve qui peut."