I've tested this myself with these results
- MA15 is ok
- Reinstallation of Witcher 3 is ok
- MasterCard and Visa both do not work.
- Visiting the Steam store page for Witcher 3 is blocked for me.
When I contacted Steam support they said specifically only "credit cards" being on file is supported. However, they indicated they would be adding additional verification methods in the future.
If people are finding success with specific debit banks and debit cards, I'm more likely to believe that is an unintended bug or something strange going on than an intended feature.