Actually, I did know about them already (actually, I predicted their existence before I discovered them in a book, based on common personalities found in recently-settled areas,) but still, the fact that I know about things like The Clearances (the migration of people out of the Scottish Highlands in the 17 and 1800s, about which much writing also strikes me as propaganda,) and the migrations of Turkic-speaking people into central Hungary and the movements of "barbarian" tribes through Europe in the Classical and pre-Classical period, but have only recently and by chance discovered a major movement of people and government in the midst of the Middle Ages in the midst of Europe suggests that something more than my mere ignorance is going on.
The answer is obvious: The whole subject got contaminated by Nazis (mis)using it to justify attacking the rest of Europe. (This would be like, IDK, Australia using the fact that it and the US both have a bunch of British Immigrants in it as an excuse to try to conquer the US.
One of the mysteries I was trying to figure out yesterday was that I knew the cities of Eastern Germany were settled relatively late (after the cities of Western Germany and within the past thousand years or so--compare to London and Paris, which I believe pre-dated the presence of the Romans in their areas,) and during the Eastward Migration, but the Wikipedia pages on bronze and stone-age peoples of Europe show plenty of Germans in the area. How could they be there and not be there?
The obvious now-solution is that they were different groups of Germans.
I'm still not clear on all of the details, like whether the old groups of Germans were still there when the new groups showed up, and if so, what the relationships between them were like.
It is quite obvious, though, that much of what we know as modern "Germany" and central Europe was shaped by these migrations (Berlin, for example, is among the cities founded relatively late, rather than by the Classical-era Germans described by the Romans), and that the Jews arrived at virtually the same time, often (if not generally) by explicit invitation by civic leaders who believed a Jewish population would be to their city's glory and benefit.
EG, from Wikipedia, "The actual history of the Jews in Speyer started in 1084*, when Jews fleeing from pogroms in Mainz and Worms ignited by the crusades took refuge with their relatives in Speyer. They possibly came at the instigation of bishop Rüdiger Huzmann (1073–1090), who invited a larger number of Jews to live in his town with the expressed approval of emperor Henry IV. In his notes the bishop wrote:
'In the name of the holy and undivided trinity, I, Rüdiger, with the surname of Huozmann, bishop of Speyer, in my endeavor to turn the village of Speyer into a city, believed to multiply its image a thousand times by also inviting Jews...' "< *Note that Speyer's Cathedral was only begun in 1032. This was a very new city.
And none of this was ever pointed out in any of my history classes. (Even, OMG, the one explicitly titled "Medieval Economics.")