Arduino itself does not sell an Ethernet shield with native IPv6 support. I even asked the support for it. This is a pity.
If someone has an idea how I can use IPv6 on an Arduino (WiFi or Ethernet), please reply!
@mark22k I suggest using a W6100 Ethernet shield. https://www.wiznet.io/product-item/w6100-ethernet-shield/
@BenderRobot Thanks for the tip! Unfortunately it seems to be sold out in the European store :-(