I don't know of many mainstream Jewish historians who would agree that antisemitism didn't exist prior to the 19th century. They would agree that racial antisemitism developed largely during the 19th century alongside pseudoscience about race in general, but that religious and economic antisemitism has existed for over a thousand years, and that the latter two also informed the development of the racial version. [2] For example, the Rhineland massacres in 1096 are generally considered to be antisemitic [3] and part of a sequence of historical mass murders of Jews that lead to the Holocaust, despite Europe not then having a clear concept of race.
1: https://en.wikipedia.org/wiki/Yellow_badge