1831, "iron rolled in large, flat plates for use in making steam boilers," from boiler + plate (n.). In newspaper (and now information technology) slang, the sense of "unit of writing that can be used over and over without change" is attested by 1887. The connecting notion probably is the permanence of the prepared plate compared to set type: From the 1870s to the 1950s, publicity items were cast or stamped in metal ready for the printing press and distributed to country newspapers as filler. An early provider was the American Press Association (1882). The largest supplier later was Western Newspaper Union.
An older name for it was plate-matter "type cast in a number of stereotype plates for insertion in different newspapers" (1878). Plate (n.) is attested by 1824 in printing as "a cast of a page of composed movable types."
WITHIN the past ten years, "plate matter" has become more and more popular among out-of-town papers, and the more enterprising are discarding the ready prints and are using plate matter instead. For a long time there was a prejudice against "Boiler Plates," but editors of small, and even of prosperous papers, began to discover that better matter was going out in the plates than they could afford, individually, to pay for. It was found that the reading public did not care, so long as the reading columns were bright and newsy, whether they were set up in the local office or in New York. ["The Journalist Souvenir," 1887]