We don’t plan to backport this feature, but if the demand is really high, we’ll consider it.
— rkFlorian commented
Well, a security feature w/o IPv6 support is totally useless as soon as you enable IPv6 for just one subscription. On the other hand no IPv6 support for subscriptions makes Plesk totally useless for any ISP serving international customers.
So I suspect the answer is yes, the demand is really high for your target customer base.