From EU Regulation 2018/302 (applies from 3 December):
Article 5
Non-discrimination for reasons related to payment
1. A trader shall not, within the range of means of payment accepted by the trader, apply, for reasons related to a customer's nationality, place of residence or place of establishment, the location of the payment account, the place of establishment of the payment service provider or the place of issue of the payment instrument within the Union, different conditions for a payment transaction, where:
(a) the payment transaction is made through an electronic transaction by credit transfer, direct debit or a card-based payment instrument within the same payment brand and category;
Let's say that a customer uses a card issued in an EU country and denominated in currency ABC, but that the merchant's local currency is XYZ.
Assume that the merchant wishes to offer DCC, allowing the customer to pay in currency ABC instead of XYZ. Is the merchant then bound, by the anti-discrimination clause, to also offer local customers with XYZ-denominated cards a choice between currencies ABC and XYZ, with the same order of preference in on-screen menus? If so, then this regulation might be what finally stops DCC within the EU (at least for EU cardholders) as local customers would be surprised if terminals were to offer payment in dozens of foreign currencies in addition to local currency.