They aren't actually doing that, as far as I've seen.

There are a number of big ISPs in Europe that that have a "new" native v6+CGNATed v4 platform, and an "old" native v4+no v6 one. That's where a lot of the "I had to ask the ISP to disable v6 to get inbound connections" comes from -- the actual relevant part was getting out from behind CGNAT, and the loss of v6 was just a side-effect caused by the ISP moving you to their old platform.

Inbound v6 should generally work on those ISPs. If it didn't... I'd be tempted (perhaps unfairly) to blame the user for not configuring things properly, or their software for not supporting v6. A lot of people will give up and blame a problem on v6 when they would have found and fixed it instead on v4.