Despite many people denying its existence, the truth is that researchers and activists have independently found asexuality many times. It hasn't been until recently that it has started to become a somewhat known word, so most of the time these writers weren't getting it from each other. It's not like us knowing what a unicorn is, not because we've ever encountered one in real life, but because we've heard other people talk of them; no, people looking at dissident sexuality were encountering asexuals again and again.
In 1869, the journalist Karl-Maria Kertbeny coined the terms "heterosexual" and "homosexual", giving them pretty much the meaning we all know today. But few people know he also included the category "monosexual", meaning someone who doesn't want to have sex with people of either gender, only masturbation.
The sexologist Magnus Hirschfield is another figure that always comes up in the history of early LGBTQ rights advocacy. He, too, wrote about people who don't feel sexual attraction (he used the term "sexual anaesthesia") in a pamphlet in 1872.
Same with Emma Throsse, the first known woman to write scientifically about lesbianism. She's most known for her 1895 publication defending the rights of homosexual people and in particular for her writings about lesbians, but she also wrote about "asensuals". Not only that, but she goes on to mention that "the author confesses to this category", meaning that she is asensual herself. (But even now, when looking for her Wikipedia page, it only mentions that she wrote about homosexuality).
In 1897, the sexologist Christiane Leidinger made the first modern definition of "asexuality".
In 1907, the activist Carl Schlegel published a document demanding "the same laws for all intermediate parts of sexual life: homosexual, heterosexual, bisexual, asexual, be legal now as they are for heterosexuals".
When the biologist and sexologist Alfred Kinsey (known as "the father of the sexual revolution") made the Kinsey scale to describe people's position in the Homosexual-Heterosexual scale (with various degrees of bisexuality), he also had to create the "Category X" for people who did not have any response that could be described as sexual attraction, because his experiments with both men and women were finding people who only fit this Category X.
And these are just a few examples. Contrary to what bigoted people say, asexuality was not "invented on the internet" and it's not a recent "trend". It's always been part of humanity, same way as it's also part of other animal species. The reason why you hadn't heard about it before is because it's invisibilized for going against the heteronormative and sex-normative moralistic views, not because it wasn't there.
