So obviously we need to clear up what I mean, “Prussian” not exactly being a unique ethnicity or even language- however, anyone who has ever met a German will tell you that their regional identities are usually much more strong than their national ones. Every German I’ve met as identified themselves as “Hanoverian” or “Bavarian” etc. rather than German.
So, what with the old Kingdom of Prussia being the dominant military (and later political) entity in the Holy Roman Empire and then German Empire, it’s reasonable to assume that they had just as strong a regional and cultural identity, if not stronger due to their geographical displacement from the rest of Germany and also their dominance in the aforementioned manner.
So with all that established, the USSR comes in, ethnically cleanses Ostpreußen and fills it with Russians, all the Prussians are deported to Germany. What happens to them though? Are there any today who keep up Old Prussian groups like there are with Rhodesians? Did any remain in Danzig? Or were they all integrated into Germany at the time? Are there any left today at all or has the culture completely gone?
I can also throw on the end that I am aware of the German “aristocracy” who are still cosplaying as having their old titles despite Germany now being a Federal Republic, I’m not sure to what extent if any these individuals maintain a Prussian identity, nor if that identity is genuine, nor if it is a widely practised identity, so my mere cognisance of their existence doesn’t help me that much unfortunately.
Edit: just wanted to add I’ve received a few comments about links between the Nazis and Prussian history, and about how there were actually a few different peoples within the Kingdom of Prussia, etc. As interesting as this is it’s not what I’m asking about, not about the Nazis or the Teutons or the fact that the Prussians were originally a Baltic speaking people, I’m just interested in to what extent the Prussian identity, to the extent there was one (that being German- Speaking people who lived in Ostpreußen and the surrounding area and referred to themselves as Prussians in the way a Bavarian would call himself a Bavarian) persisted after their expulsion from their regional homeland following its annexation by the Soviets.
This comment is mostly coming from the book Iron Kingdom: The Rise and Downfall of Prussia, 1600–1947
Which was actually recommended to me by this subreddit over 10 years ago:
After world War II, an idea formed that the development of Germany did not follow a similar paths to the other nation states of Europe and that their path through the kingdom of Prussia, which had formed the German empire, was partly the cause behind the rise of Nazism in Germany. The idea was that while other nations such as France or Great Britain liberalized and threw off the militant aristocracy to become modern nations, the institutions of the Kingdom of Prussia were ones that inserted a profound sense of militism into the nation of Germany, so that even in the Weimer Republic when the kingdom was just another province in the nation (albeit the largest one), its influence gave rise to the Nazis. I dont believe the argument is as 1 to 1 as the history of the kingdom of Prussia gave rise to the Nazis, but more that it was a negative influence.
With this argument in mind when the Allies occupied Germany after WW2, they dissolved Prussia as an administrative devision.
To then add more answers to your question around identity, the kingdom of Prussia before the unification of Germany comprised an number of states through out Germany as well as areas of modern day Poland. The form of the kingdom was not as consistent as a region such as Bavaria and so it actually occupied a space where it contained multiple people identities, which probably also contributes to the lack of identity.
Finally the name Prussia itself is kind of an odd historical creation in that Prussians were a non germanic group conquered in the Baltic crusades and through the secularization of the teutonic order came under control of the dynasty of the elector of Brandenberg which eventually united together under 1 person, and then was transformed into the kingdom of Prussia (the process would deserve its own answer).
References: Iron Kingdom: The Rise and Downfall of Prussia, 1600–1947
The kingdom of Prussia wasn't the dominant state in imperial Germany; imperial Germany was Prussia with some other bits glommed on. So it doesn't make sense to talk about a Prussian "regional identity." There were a whole lot of heterogeneous (and sometimes non-contiguous) regions within Prussia itself, several of which were largely coterminous with their corresponding contemporary Germany states (Brandenburg, Hessen, and Schleswig-Holstein are mostly the same; NRW is basically Westfalen and the Rheinprovinz jammed together).
Now, Ostpreußen and Westpreußen did have distinct regional identities, but bear in mind that they were just two of as many as thirteen provinces, and were mostly rural, pretty sparsely populated compared to the western provinces, and, in the case of Westpreußen specifically, only about halfway Germanized. Ost- and Westpreußen combined had less than 10% of Prussia's population in 1910. And not only were a lot of people in Westpreußen Polish or Kashubian, but a lot of people in Ostpreußen spoke Middle German dialects, not Low German like the majority. So we're really talking about a half-dozen or more micro-regional identities now (as you noted with your remark about Hanover, identity is sometimes fixed strongly to cities, and that was especially true of the old Hanseatic ports in East and West Prussia—Danzig, Königsberg, Elbing).
Finally, I'd caution you to take the idea of deeply seated regional identity with a big grain of salt; it's a modern notion that would have been pretty incomprehensible to the average peasant even in the 19th century, and certainly before then (for most people through most of European history, the entire world was a few villages, a market town, maybe a pilgrimage site; the duke or king or whoever may as well have lived on the moon). The singular Bavarian identity we're familiar with today, for instance, was artificially assembled in the late 19th century from a vast, extremely diverse array of folk traditions.
All that said, and with the annoying caveats that 1) almost everything's only in German and 2) I am not a native speaker and will find it too laborious to translate much of this: Yes, the East and West Prussian identities still exist, after a fashion. They are promoted by groups sometimes uncomfortably similar to those lost-causer Rhodesians, which include some youth organizations, so they're at least trying to maintain their identity into the 21st century. Very few Germans, if any, remained in Gdańsk; it seems like the East Prussians expelled in 1945 settled predominantly in the north and northwest (although I did see a couple references to Bavaria being the "Patenland" of East Prussia, which is like…godparent-state?).
The Landsmannschaft Ostpreußen has text and photo archives (with an English site, in the latter case), a weekly newspaper, and a youth wing.
There are a couple museums dedicated to East Prussian history and culture: one in Lüneburg and one in Ellingen (which is in Bavaria). Both are supported by a foundation that doesn't seem to have its own site, but is described (in German, naturally) here on the LO site.
There's also this news page, which is run by…some guy? Lots of sites like this scattered around the internet, but this one is notable for being up-to-date; others I found had been defunct since 2008, 2004, even 2000.
Then there's the Westpreußische Gesellschaft, which sporadically awards cultural prizes and publishes both a sort of almanac and a magazine. Per Wikipedia, four different publications have merged into the latter in the past couple decades after having been independent all through the 20th century; as the prewar generations die out, we might be nearing the end of these efforts at cultural continuity. (Also interesting: three of the four were rooted in specific cities, namely Danzig, Elbing, and Marienburg.)
Finally, at least for now, there's another Bavaria-based foundation representing both East and West Prussia. This one is in Polish as well German; I noticed Polish and even Russian options among these sites, which suggests an orientation toward the people who actually live in the regions today. There's some revanchist language here and there, but the vibe in most of this stuff (not just this last site, but everything I've been looking at) seems genuinely more about outreach and preserving history than grievance or vengeance.