I was introduced to this phrase by Jerry Seinfeld (well, technically, Elaine Benes), but Wikipedia actually says that it's older:
The ["Yada yada"] episode is one of the most famous of the series, specifically for its focus on the phrase "yada yada". "Yadda yadda" was already a relatively common phrase before the episode aired, used notably by comedian Lenny Bruce, among others. The phrase may have originated with the 1950s "yackety-yack", 1940s vaudeville, and earlier.
Wiktionary has this:
American, origin unclear, perhaps onomatopoeic of blather; perhaps adaptation of dialectal speech, perhaps from yatata or yatter. Various variant forms appear in the US 1940s–60s; for example, the 1947 American musical Allegro by Oscar Hammerstein II and Richard Rodgers contains a song called “Yatata, Yatata, Yatata,” about cocktail party chatter[...]. Popularized in the United States in the late 1990s by TV show Seinfeld[...]. Sometimes popularly attributed to Yiddish, but this is dismissed by etymologists[.]
As to the meaning, it's a (colloquial, informal) way of saying "and so on, etcetera". In your particular case, what is being implied is that when the data dumps are released, there's usually some text in rather standard wording, and if you're a regular, you begin to ignore that text after a while. But this time, the data dump is accompanied by some additional, unusual announcement (namely, "we’re moving to a bi-monthly schedule instead of monthly"), and the author warns you in advance: watch out, this is not the usual blah-blah, or the usual wall of text, so don't ignore it.
