Super rare is untrue. Rare would even be a stretch. I live in Copenhagen and use a helmet. I would claim it is common to use a helmet. Not everyone - but many do. And it is not even mandatory.

Helmet usage is actually increasing: https://www.sciencedirect.com/science/article/abs/pii/S00224...

We recommend it for tourists as well. Though it is rare with helmets for that group who might need it the most: https://www.visitcopenhagen.com/sites/visitcopenhagen.com/fi...

Helmets are not compulsory and interest groups works to keep it that way: https://www.cyklistforbundet.dk/english/use-of-helmet

But less scary F150s and good bike infrastructure, yes.