I will be blunt at the beginning: I don't have a definitive answer to your question. But it is absolutely what I would expect to be true. Contemporary sex researchers often talk about "sexual scripts" or "sexual repertoires". And it seems like this wasn't really part of the sexual script in the 1940's. That doesn't mean sex workers wouldn't engage in it, it just means we shouldn't assume that it's a "natural" part of an uninhibited sexual experience. These sorts of sexual behaviors are things that are fundamentally learned.

We have one survey from before Kinsey (it's of married middle class women in 1920's, made by an academic just for her own reference) but in that survey only 7% of women had had pre-marital sex and of that 7%, about half had only had pre-marital sex with the man they were later married to. The culture just had a lot less information about sexual contact in it.

“Petting” or "heavy petting", an ambiguous term that covers everything from passionate making out to oral sex (sort of like the contemporary “hooking up” covers a wide range of behaviors ambiguously) only arises in the 1920’s with modern dating, rather than courting. Even as late as the 1910’s you have some thought leaders calling for women to refuse all intimate physical contact, even hand-holding, before marriage (or at least before engagement). However, too much can easily be made of this, and some things I’ve looked at seem to suggest there was more physical contact in actually existing 18th century courtship than in the ideal, theoretical version. This pre-marital contact seems to primarily hand-holding and perhaps kissing. One the most interesting books I've read on the topic is the oral history *Sex Before the Sexual Revolution: Intimate Life in England 1918–1963 * and, based on that, I expect much of the kissing was “dry kissing” rather than “French kissing”. Which is so odd for us, because for us "French kissing" is a cornerstone of our modern sexual script, modern sexual repertoire. But actually, a large number of cultures worldwide don't kiss at all romantically (see my old answer Did people always 'make out'? How was attraction expressed physically throughout, for example, early colonial USA?). Specifically on oral sex, Sex Before the Sexual Revolution has it almost non-existent in the couples interviewed before the sexual revolution.

The rise of petting and dating, historians have argued, is linked to cars and unmarried young people being able to create more privacy (I think both Beth L. Bailey, From Front Porch to Back Seat: Courtship in Twentieth-Century America and Moira Weigl, Labor of Love: the Invention of Dating point to this, but it's been a few years since I've read them). In the famous Kinsey Reports (there were two of them, the male one published in 1948 and the female one published in 1953), oral sex was characterized as "the last petting activity to become accepted". I don't have the Kinsey books in front of me, but one paper I'm looking at cites "Kinsey et al. (1948, 1953) presented data on the premarital sexual behavior of both males and females showing different rates for giving and receiving oral-genital stimulation. Both sexes were more likely to report having received oral stimulation than to report having given it. Fellatio was more often reported than was cunnilingus: 17% of both sexes reported fellatio, and 11% reported cunnilingus". People tend to agree that some things Kinsey’s non-random methodology overestimated (homosexuality, for one), and while these aren’t perfect numbers, I think they’re likely in the right ballpark.

We don't have a national survey until the 1990s and the AIDS crisis, but going based off of small, non-random samples from Kinsey until the national surveys summarized in Newcomer and Udry’s 1985 article "Oral sex in an adolescent population" Archives of Sexual Behavior, it seems like through the 1960's oral sex was reported notably less commonly than intercourse. It is only over the course of the 1970's and 1980's oral sex's prominence seems to rise. We can think of it as a third wave: first petting starting in the 1920's, then the canonical sexual revolution of the 1960's, and only later oral sex. This article ties oral sex's rising prominence in this period, at least among adolescents, to a desire maintain a technical virginity. I think the evidence isn't great either way, but it may make sense to think of oral sex's place in the sexual scripts of this period as Kinsey does, as not a form of sex that's a pleasurable and equal alternative to intercourse, or as a part of a full and expressive bedroom experience, but as a form of "heavy petting", a way to have sex without having sex—though this still does seem to be a step towards sex. In Newcomer and Udry's pooled samples of various surveys of college students of the period in the late 1970s, 1/4 of virgins had given or received oral sex and 4/5 of non-virgins had.

So, again, I don't have clear data about sex workers in World War II in particular, but oral sex just doesn't seem to have become a typical part of the Anglo-American sexual repertoire/sexual scripts until perhaps the 1970's and so, while I can't speak to "rare", it would not surprise me if it were also just not what sex workers commonly performed or had performed on them, either. Sex workers are less constrained by norms, certainly, but they still exist within the norms and expectations and quite frankly the tastes of the society they live in.