How about French, German and Italian culture groups?
I didn't use them as examples because I don't know much about their cultural history and would have a hard time telling what's the difference between Westphalian and Saxon or Lombardian and Neapolitan.
Time to read some scientific papers? Here's a good one:
A. M. Sakharov, "Rus' and Its Culture in the Thirteenth to Fifteenth Centuries,"
Volume 18, Issue 3, 1979
It deals with the effects of the Mongol invasion on the forming of the Russian nation. The culture is always described as "Russian" (with a footnote about Byelarusian and Ukrainian cultures), and I wasn't able to find any mentions of Novgorodian or Ryazanian cultures being significantly different from Muscovite. In fact, Ryazan is only mentioned in passing as "an important centre" and Novgorod is mostly discussed in terms of politics and the creation of the republic. One aspect might be that Novgorod wasn't affected nearly as badly by the Mongol invasion, and so it was better able to preserve the Russian culture therefore being an important city for its resurrection.
By the end of the XIII century there already began a "renaissance" of Russian culture, and after the unification of Russia there was a further "interpenetration of local cultures", but it's never suggested that the difference between Muscovy and Novgorod was just as big as between Muscovy and Southwestern Rus (aka Ukraine - called the Ruthenian culture in EU4). By this logic, you could assign a separate culture to every Russian state. Tver, Nizhny Novgorod, Pskov, Smolensk, Chernigov, Pereyaslavl...
So auto-updating on Steam will just make all your saves unplayable? Sounds awesome.
You can go to "betas" settings menu and choose the previous patch to finish your games. Just don't try to open any 1.15 games in the 1.16 version.
That will apparently break them forever.
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