Not sure were you were. I am from northern Germany, around Hamburg.
But every single store that does accept cards (not every little store can bear the fees, though) has theses signs.
So I would counter your n=1 anecdotal argument with an equally non representative n=1 argument. No one learns anything, except that some shops in Germany ask for the use of cash-alternatives, others don't.
On a recent trip to Germany, I didn't see a single such sign.
Germany is changing gradually, perhaps due to the EU's limits on the fees Visa and MasterCard can change, but Covid-19 doesn't seem to have much effect.
But I asked someone in a popular local store ( Belgium) and consumer behaviour hasn't changed.
Also, you mention Covid would be a stimulator for digital payments and that doesn't seem the case in Belgium. So I wouldn't know why it would be the case for Germany.