I've never heard of Switch, but to my knowledge there is nothing about Maestro that integrates specifically with the German banking system. It just so happens that German debit cards are typically co-branded as Maestro (and nowadays sometimes V-Pay), which is why it was commonly accepted. The fact that fees were lower than credit cards probably played a significant role in that.

When the EU capped the interchange fees in 2015, many retailers started accepting MasterCard and Visa here. AMEX is exempt from the regulation since it's a three-party system and less commonly accepted. I'm not aware of any retailers that accept MC/Visa debit, but not credit cards or vice versa.