[HRT] is a recent development right?

Depends on how you define recent. The earliest studies on the effects of "cross-sex" hormones happened over a century ago. I think there was some stuff done before him, but the big name in that area was an Austrian doctor named Eugen Steinach. In the early 1900s/1910s he experimented with implanting rats and guinea pigs with ovaries (into males) and testes (into females) and observed the corresponding physical and behavior changes. Sex hormones had not been isolated at this point, so it was only understood that these sex organs contained what they termed "glandular juices" which was having these effects. In 1923 he produced Der Steinachfilm (trans. The Steinach Film) which, along with a 68 page pamphlet published in 1920, popularized his discoveries to the lay public. These results were interpreted in some crank ways, like the belief among some people, including Steinach himself, that homosexuality could be "cured" by giving gay men a testicular implant to boost their "male glandular juices" (testosterone), but the trans implications were picked up by other people.^[Robert Beachy, Gay Berlin: Birthplace of a Modern Identity, (Random House, 2014), 173-175.] Apparently a trans woman read about these developments and wanted the prodecure for herself, quoting from Gay Berlin with editted pronouns,

The pamphlet was mentioned by one young [woman] who wrote [sexologist Magnus] Hirschfeld in 1920—a rare surviving letter preserved at the Kinsey Institute—explaining that [she] was "more woman than man." This correspondent also described that [she] had just had the chance to read about "the book about artificial rejuvenation and sex change," a clear reference to Steinach. The promise of hormone therapy inspired [her] to write Hirschfeld: "[W]ould it not perhaps be possible to remove my testicles and in their place insert an ovary, and then in this fashion I could become a complete woman. . . . I trust you completely Herr Dr., and perhaps I could serve science as an important experiment." Included in this missive was a small photograph of [the woman in women's clothing].^[Ibid., 175]

There are other reports of people receiving a very primitive version of HRT around this time as well. In 1916 a 36 year old person known only as "Herr. A" in the historical record approached a Berlin doctor named Max Marcuse about an orchiectomy and hormone treatment. Marcuse denied the request for surgery on the grounds that he deemed the patient mentally ill, but gave them an unspecified "ovarian preparation" which seemed to calm their distress. One of the first surgical sex reassignments was undertaken in stages over the course of 1920-1921 on a 23 year old military officer. They received an orchiectomy, a primitive sort of vulvoplasty while keeping the penis intact, and an implanted ovary for hormone treatment. Though this person later requested the reversal of some of these surgical procedures and then lived out the rest of their life as a man.^[Ibid., 176-177]

According to Julian Gill-Peterson, synthetic hormones were available in the United States by the mid-1930s, though trans people's access to them were obviously extremely difficult, if not impossible, though not for a lack of trying. Gill-Peterson relates a story of a trans man known to us only as "Bernard" who was well read in the sexological developments of the time and attempted to convince doctors at John Hopkins Hospital that he was actually intersex and needed surgery and hormones to "correct" his supposed birth defect. Unfortunately he was ultimately unsuccessful, though he did manage to fool the first doctor he saw, but that doctor sought a second opinion and Bernard's ploy was seen through by the second one.^[Julian Gill-Peterson, Histories of the Transgender Child (University of Minnesota Press, 2018), 83-85.] So that all said, the most primitive forms of HRT have been around for just over a century now, and synthetic HRT has been possible but not nessicarily available since roughly the middle last century. I don't have any citation for this but I remember hearing way back in the day that spironolactone was originally prescribed as a blood pressure medication for men in the 1950s until they all started growing breasts because of its anti-androgen effects. So the past 70 or 80 years have mostly been about increasing access moreso than novel developments in HRT.