Using
https://www.worldometers.info/coronavirus/country/germany/ and a broad thumb, I'm estimating roughly these case fatality rates for the different waves:
W1, Feb-May 2020, 180k cases, 9k deaths allowing for a few extra weeks --> 5%
W2, Oct 2020-mid-Feb 2021, 2M cases, 61k deaths --> 3%
W3, mid-Feb 2021-May 2021, 1.5M cases, 20k deaths --> 1.3%
W4, August 2021-mid-Dec 2021 [ignoring recent cases because their outcome isn't fully known yet], 2.9M cases, 20k deaths --> 0.7%
I think the attribution of deaths to cases by shifting the data by a few weeks is accurate enough given the large number of samples the recent waves have provided us with. W1 was under-tested for cases, no idea by how much, and treatment may have been less ideal back then so I'll ignore W1. Test coverage between W2 and W4 hasn't really changed much, and breakthrough medications haven't really hit yet.
Going from 3 to 0.7 is a significant improvement.
What changed?
Lots of people got vax'd.
Edit: This is vax'd and unvax'd people's data mixed together - plenty sources confirm the vax-only data is another significant step better, the roughly 4x reduction in overall death rate is already achieved with the poor German vax rate.