akuperma > movements towards intermarriage and people going OTD in large numbers began about 250 years ago.
this is true for Western Europe and US (Mr. Harris). For most of Eastern Europe and Russian Empire (Reb Zelinsky), intermarriage started in 1920s. Before that, Jews lived behind the “pale” and even those who were professionals and allowed to live in big towns, still married Jews. Even in Germany late 19th century, Jews that baptized would typically marry people with similar yichus.