A handful of policies do not have a dedicated 'ring exclusion.
But even on those, if something went titsup the insurer could come after you using the 'prepared course' exclusion present in all policies.
All insurers have to cover you third party on European roads by default.
What this means is that if you stacked it and there was a third party claim the insurer would pay out and then come after you to recover their losses.
Of course you might not stack it.
But are you willing to risk your insurer coming after you for a few hundred thousand after you've side-swiped a super-duper 911?
Far better to go out and go a track day with someone like Destination Nurburgring and not worry about idiots and your insurance.
As DH said.
I looked into Ring specific insurance about 4 years ago. Back then I was taking a 1.6 MINI Cooper with 120bhp (so no exactly a top end racer) and they asked for £4500 for a maximum payout of £15,000 to myself (they would obviously pay more to any other parties involved). I actually found a better deal in the form of hiring a car there.
I paid in total 300 Euros for 48 hours in a (practically brand new) Renault Clio 197 Cup with a distance cap of 100km (enough for about 5 laps of the Ring IIRC). There was a 3k Euro deposit (accepted credit card) just in case things went tyres up, but apart from that there was just fuel and lap tickets to pay for.
Looking back I couldn't think of a better package really. Overall the pricing was very reasonable. The Clio Cup was also the ideal car for my first laps on the track as the relatively small power output (especially being NA) meant the car was very predictable, balanced and equipped with sufficiently capable brakes. The distance cap also meant that I limited myself to 2 laps on the first night and 3 the next, which I'd also advise as the Ring can definitely carry you away. Forcing myself to take a break prevented any red mist descending and is perhaps one of the biggest reasons why I didn't get into any incidents.