Yes, the "fire" quote is just a rhetorical device from Schenck. No, the "fire" quote is likely not true today.
The supreme court has very narrowly defined exceptions to the first amendment since the time of Schenck (just "fighting words," which is vague and narrow enough it may not really exist either). You can yell fire in a crowded theater; it's not fighting words. It just makes you a dick.
P.S., the historical context for the "fire" rhetorical device was that there had been a series of horrific theater fire mass-deaths in the early 20th century due to doors that were either locked, or opened inwards — before the era of fire safety regulation. Some deaths were due to crushing at the doors, rather than actual fire or smoke inhalation. Modern theater doors open outward and don't lock from the inside, so we don't have this particular flavor of horrific incidents in our social consciousness.
The podcast you linked to in your other comment seems to think the quote is true today (although not useful in most discussion due to being too obviously true):
> It’s a rhetorical device to say the First Amendment is not absolute, which is true, but that’s not in dispute. [...]
> It’s as if we were looking at an animal on the side of the road trying to decide what it is and I said not all animals are cats. Your reaction would be yes, thank you, I am aware, but the question is, what is that animal? We have the tools we need to decide what that animal is on the side of the road. With free speech those tools are a 100 years of Supreme Court cases.
There are people still being crushed to death at events today, maybe not theaters:
https://en.wikipedia.org/wiki/List_of_human_stampedes_and_cr...
Moreover, I think this is true of most rhetorical devices in general, and that is part of what the podcast is criticizing. They are not attempts to collaboratively consider the evidence in order to arrive at shared conclusions informed by the knowledge of all parties; they are attempts to influence whatever conclusions may arise, by hook or by crook, to benefit the interests of the utterer, and whether either the utterances or the conclusions happen to be objectively true or false is none of the utterer's concern.
I consider this contemptible, and I try to engage in it as little as I can manage.
† Incidentally, this page explains that Schenck was not specifically overturned until Brandenburg in 1969 (although the Brandenburg decision doesn't mention Schenck), but the Supreme Court's progressively broader reading of the First Amendment was already protecting antiwar and anticapitalist activism by the 1950s, for example in Gibson (1946), Estep (1946), Dickinson (1953), and Yates (1957).
I understood Holmes to be saying that protesting the war in that epoch could lead to actual violence in the US. The domestic terrorism of the anarchists earlier in the decade could make you feel that way.
However, your reading of Schenck is incorrect.
I use many figures of speech every day which I don't know the etymologies of. I don't think it's necessary to study the origin of every figure of speech before using it.
The quote you know dates to 1919 and was a metaphor used to justify suppression of political speech during WWI.
https://www.oyez.org/cases/1900-1940/249us47
It's pretty fucking shameful.
https://www.popehat.com/2018/06/28/make-no-law-episode-seven... for a great podcast about it.