My assumptions would be entirely informed from extrapolating from historical context and not knowing anything about German.
So there was probably a lot of linguistic diversity before unification in 1870, then there would have been a standardization effort started by Bismarck (favoring the dialect predominantly spoken in Prussia) which would carry through WW1, would be relaxed during the Weimar republic, would intensify (or turn into something bizarre and Orwellian) in the Nazi era, and then a slight divergence between East and West Germany in the Cold War.
Under this, my rough hypothesis would be that German has actually changed a lot less in the post-WW2 era, especially since the 90s, than it would have in the period before.
Is this roughly how things shook out? I'd be really interested where this is completely wrong.